I can see your point regarding Dissidia being not canon, but the problem is that Square-Enix hasn't yet confirmed it to be non-canon. Until then, they basically hint that it's up to the player to decide.

In my opinion, I think that the storyline of Dissida is no way connected with the main Final Fantasy storylines. However, I really think that the features, looks and personalities of characters in Dissidia were supposed to originate from thier main storylines. I think it's fair to say that Dissidia is not 100% canon.

Square wanted EX forms to be recognizable to fans
Yeah, but I hardly felt that Griever was easily recognisable for the most fans in general. Griever wasn't exactly a big thing in VIII. I'd say that Griever is more recognisable to R=U fans.