Well the amount of legendaries being released every gen is increasing and it really shouldnt, 1st gen only had 5 including Mew and whats so Legendary about the Pokemon when theres over 20 now and way more to come plus theyre not going to out do Pokemon they call gods like Arceus.
Legendary describes each of them individually yet all of them group. They are "Legendary" cause generally they each:
1: Can only be caught once without cheating or trading or special events ( I don't count starters in this cause a starter is more likely to be traded with/for then a Legendary )
2: In relation to the Above there is generally ONLY 1 of each PER GAME ( I.E. You WONT Find 2 Ho-Oh in G/C/HG )
3: Some are even harder to get not even being a part of ordinary un-modded game play ( Mew & celebi for example )
4: I liken Legendary Pokemon to real life Legends. In the Real world there are prolly dozens if not hundreds of Legends, Myths, and Urban legends around the world. Legendary Pokemon are an in game paradox to that. Hence more and more are bound to be either added into the game or at least become known to the Fanbase.
Creativity is pretty easy for a fresh mind but most designers for Gamefreak are the same since gen 1 so they continue to try new things which is ruining it for hardcore fans, 1 and 2 were mostly designed at the same time hence the similarity in the look but when 3rd gen came they wanted to try new things same with 4th.
Well that could be solved a bit by them bringing fresh faces or new blood onto the team ( *cough* Nillz *cough* some moders and devlopers around here *cough* ) but clearly they don't seem ta wanna do that for whatever reasons. However to be fair based on the X & Y Trailer at least give em some credit. X and Y seem relatively cool, Bring some nice new things to the table, has potential & seems to be making/forcing them to think a little outside of their old box