To talk about the nature of either "monster rallies" or their villainous parallels, one has to ask first, "Is there any validity to the crossing-over of characters not explicitly created to occupy the same mythos/universe?"
There's certainly a long-standing critical opinion that finds that any crossovers, particularly those not initiated by the original author(s), vitiate the original concept. And most authors would agree that their original versions are the best. If you could somehow arrange to let both Bram Stoker and Mary Shelley view ABBOTT AND COSTELLO MEET FRANKENSTEIN, it's likely neither author would recognize Universal's versions of Dracula and the Monster as having anything in common with the original creations.
However, that's not quite the truth, for no fictional work-- novel, short story, film-- is a seamless whole, born as it were from the author's pure inspiration as Athena from the skull of Zeus. Every fiction is parented not only by its living author but also by other fictions which the author has absorbed. And, by the same token, every fiction is capable of birthing new "children," even if none of the parents want to acknowledge the offspring.
Take the relationship of the Shelley "Frankenstein Monster" to the creature brought forth by James Whale's 1932 film FRANKENSTEIN. Many critics have commented that the monster of the film is totally unlike that of the Shelley novel, for the latter has a Romantic locquaciousness while the former merely grunts and growls. And yet, this isn't the whole truth, for Shelley's creature doesn't start out as articulate. Indeed, Mary Shelley asserted that the whole concept of the novel flowed from a single nightmare in which she envisaged the creator of some horror awakened from sleep by his mute creation standing beside his bed-- a scene which Shelley does indeed work into the novel.
Now, James Whale may have had any number of practical reasons for making his version of the Monster mute aside from some animalistic growls. But the point is that his incoherent monster does mirror one stage of the character's development within the Shelley novel. Moreover, in both works that stage of development is symbolically comparable to an aspect of experiential reality. The Monster, though created from the charnel-house rather than by human concupiscence, is in both works an uncomprehending child in a giant's body. Thus with this story-element, which I might term a "mythologem" since it enhances the mythicity of the narrative, Whale proves that he is in essence faithful to one of Mary Shelley's most important concepts, even if he diverges from others.
This sort of divergence, which Harold Bloom calls "misprison," is clearly inevitable. No later creator ever makes an entirely faithful rendering of another author's work, for the later author always changes some things for his own satisfaction or for the satisfaction of his audience (or both). In some cases, we don't know all the details of the original's work-- like how Shakespeare would have staged his own play HAMLET-- but we can be fairly sure that it would not be identical in all respects to a staging by Kenneth Branagh, even if both used all of the exact same lines in the text.
Now, given that misprison is inevitable, the sins of the children may seem no greater than those of the fathers, who were, in their time, children also.
Now, Whale's FRANKENSTEIN is not a crossover, but an adaptation. But it should go without saying that every crossover is in some sense an adaptation, even when said crossover is engineered by the author of two creations who did not originally plan to associate the two but later decided to "adapt" them into a common universe (see my earlier examples of Haggard and Burroughs). And even if one agrees with the view of some critics that the original Shelley novel is superior to the Whale adaptation, it should be a foregone critical conclusion that this assertion does take from Whale's work of all claim to quality. One should be able to make this logical conclusion even without knowing the critical consensus on the work, which, in the case of the 1932 FRANKENSTEIN, is that it's generally considered a classic (though sometimes only a minor one, depending on the critic).
Now, keeping to the notion that the Shelley novel is superior to the Whale adaptation, it might seem logical to some critics that a "monster rally" type of film is going to be inherently inferior to both. But that isn't necessarily the case. Crossover concepts may, as much as the Whale film, borrow mythologems-- or "tropes," if one prefers a more theory-neutral term-- from the original work, and may yield interesting formulations by comparing and contrasting the mythos of one concept with that of another.
The 1944 film HOUSE OF FRANKENSTEIN is a "monster rally" film which does this, for all that no critical consensus is ever likely to rate it up there with either the Shelley or Whale works. But it does do exactly what the Whale film does: abstract aspects of the Frankenstein Monster concept and then crossbreed them with those of another concept. The concept I reference is that of the Wolf Man, cinematic creation of Curt Siodmak, who provided the story for the film but did not write the script, credited to an Edward T. Lowe, Jr. As for a scene that best shows one of those interesting formulations, this takes place at a point in the film when the film's mad scientist Dr. Niemann has revived both the Monster and the lycanthropic Larry Talbot from a frozen tomb. The Monster comes off worse for wear than Talbot, and seems on the verge of perishing, so that Talbot, still full of guilt for the Wolf Man's killings, comments:
"[The Monster] wanted only life and strength-- and I wanted only death. And now look at us--"
Now, some may find this line melodramatic, and I make no claims that it will last the centuries. But on its own terms it's a very good line that sums up the irony inherent in pairing two such different monsters-- one representing "too much life," in the sense of a monster that ruthlessly slays anyone it sees for the sheer love of killing, and one representing "too much death," in the form of a creature assembled from dead body parts.
It may be fairly stated that the rest of the film does not follow up on this insight, and I agree. HOUSE OF FRANKENSTEIN is meant to be a thrill-ride, with lots of absurd furniture-moving going on in order to justify lumping together the Monster, the Wolf Man, Dracula, a mad scientist and a hunchback. But even a small nugget of gold is still gold, and so HOF is appropriate for my purposes to show how even a work aimed more or less at an audience in search of simple thrills is still able to deliver a worthwhile insight, given birth specifically from the association of two character-concepts not originally meant to be played off of one another. This life/death mythologem is not radically different from the sort of conceptual insights one gets in greater profusion from, say, the Moore/O'Neill LEAGUE OF EXTRAORDINARY GENTLEMEN graphic novel-- but there are a lot of other pleasing qualities in LOEG that are not purely about the pleasure of the crossover-text. HOF, then, fares much better to show how that specific pleasure works by virtue of being a much simpler text-- and I think makes a better proof than the more complex work does, as to how worthwhile the pleasure is in itself.