This is where "counts as" does wonders.
Trust me, I would ABSOLUTELY prefer that and used to suggest it. You model building guys like to be "creative", "different", and "original" though.
I think there is room for creative, different, and original. The problem arises when we allow one person to do their thing. It sets a precedent.
Personally, I have no problem with Mike's Aqua Marines. I like the models a lot, I like the way the datasheets were handled (critiqued by this community), and I think they made for a very cool addition to the event. The fact that they have been present in at least of of the 3 Megabattles does hurt either. But now we've got people like Alex Moon, who want to field "Blood Ravens", a chapter out of a video game. This chapter apparently can be good guys or bad guys, has fan generated datasheets, special characters, etc. Do we let that in? Surely someone could justify us doing so if we let the Aqua Marines do their thing. I happen to view the two armies VERY differently. Allowing Moon to use his Blood Ravens as "Blood Ravens" and take the fan generated datasheets, special characters, etc. sets a precedent that I strictly DO NOT want set.
This sort of thing is viewed in many different ways by different people participating in the event. It puts us in a crappy position. Whatever the outcome is people end up disappointed.
It would be much, much easier to use "Counts As" for everything not in Apoc, Apoc Reload, or IA.
Is the potential for "cool" greater than the ease of "Counts As"? Certainly not from where I'm sitting. I'm sure Paul would disagree though, and if done right in a small number of instances, I can see myself reconsidering... which is why we leave things open ended...
What got played in last years Megabattle that was not out of Apoc, Reload, or IA? Just the Aqua Marine datasheets?