You know you don't need event legendaries to complete the Dex anymore, right? So that shouldn't even be an issue.
I'm more concerned with the fact that they've had too many BW2 exclusive events. I can understand limiting it to the most recent generation of games, that makes sense because of communication limits. But the same events for BW2 work with BW1 (with the exception, of course, of new forms introduced in BW2), so there's no real reason to do this. Meanwhile, keeping to only the most recent games adds a sense of inflexibility to this system, forcing you to buy the games as soon as they're released to not miss out on them.
Now, I can understand the reasoning behind this to some degree. Of course they're going to have some sort of early event to encourage people to buy it sooner rather than later, but that may not be possible for some people, but in the event that you can't buy the games in time (for instance, if you're too young to work or unemployed), it's a bit unfair if that's your only opportunity to get a particular Pokemon. *coughGenesect*
The DW was even worse in this regard, because once BW2 came out, BW1 lost almost all support for DW promotions and many DW skins and dolls were only available within the game's launch window. Pretty much, if you didn't get BW2 the day it released, you missed out on a lot for no real reason.
In general, they do an okay job with the events, I think, but there's still a few problems that need to be sorted out.