There are also so many Pokémon only available by Wi-Fi events that don't repeat, so the ones with bad luck, like me, won't really be able to get the legendaries unless we go on the GTS and trade away our other legendaries.
Since I bought Diamond so late, I missed out on everything. I missed the first few events for White even though I bought it on opening day because I traded in my DS beforehand to get credit on my 3DS. I managed to get the Victini event though. I keep an open eye for them all now.
I really want a Mew, but it'll be extremely difficult to get one. Too difficult, that it's just demotivating sometimes.