The reason for event legendaries is so you don't become overpowered in the game just by getting something like Genesect or Keldeo half way through your game play and proceed to devastate all the trainers, gyms, E4 and Champion.
There's a reason why Pokemon like Keldeo are banned from the PWT, simply because they're too overpowered (look at Keldeo's sp. atk stat). I know that Reshiram and Zekrom are really strong too, but their the title/ cover Pokemon for Gen V so they have to be available in-game. Gamefreak have got to at least throw us a bone or two in terms of obtainable legendaries.
Additionally, there's also the "special" element to these event legendaries, hence why they're in Cherish Balls. If everyone could just catch every legendary in every game, what's so special about them anymore? Pokemon such as Mew, Jirachi, Celebi and Victini are incredibly popular in terms of design and usage. They're cute and they pack a punch in battle. That's why they're rare event only Pokemon, in order to promote people to participate more actively in the franchise, promote trading with each other and (probably) to stop you from just restarting your game and getting all the legendaries again.
Also, all Pokedex completion in Gen V does not need any event Pokemon to make it complete. You don't need Meloetta, Keldeo, Genesect etc. to complete your dex.
