What is fascinating about this "debate" is that it is the confluence of several factors that have never been part of the classic comic collecting milieu: (1) The desire to have your slabbed comic say "1st appearance." In the old days, the market just sorted itself out - now people want to have the comfort of those little words on the right side of their slabs; (2) The instantaneous nature of information dissemination. Again in the old days, people would do research, write or read articles in Overstreet, CVM, CGB or *gasp* Wizard and again, it would, over time, sort itself out - now it is like an immediate blood chumming shark tank for every new character announcement; and (3) Media hype price explosions. You take all of these together and first appearance becomes more than what it has always been - the first appearance of the character in the context of a published, serialized comic book story. Now it is more of a "dive for the last of the musical chairs" type cash grab where you just keep going back to find something else that can be hoarded and cashed out when the character makes a cameo in "Devil Dinosaur 4."
Historically, a first appearance is what it is, and don't forget, there has always been that old chestnut about "cameo appearances" a la Hulk 180. There can't be a cameo unless the character is already known, and thus a first appearance cannot be a cameo. So anyone that wants a first appearance to be anything else is going to be bucking the trend of historic collecting parlance, which is fine. Just don't expect the old guard to change. Personally, I wish those folks would find a new term and leave "first appearance" alone - call it a "Bingo!" or a "money shot" or a "dead president bonanza" or whatever.