tl;dr: No event Pokemon are required for any 'Dex completion. Use the GTS wisely and it'll go by quicker than you think. Always have an online 'dex handy for what you need and if you put the time in, you'll get there.
I've got a few but I'll be honest, I wouldn't trade them even if someone did promise me they'd trade 'em back. They were such a bitch to get for me; I've got Ruby/Crystal/Fire Red/Platinum and Black 2 but didn't have any files already created since I was planning on selling them way back when (and the nostalgia monster kicked me in the nuts), but have no old Gameboys. So I had to borrow one and haul ass on getting what I needed (which was basically all the legendaries). Anyway, my point in all this is if you want it bad enough you'll find a way. Such is the world for an aging Pokemon Master.
Most of the requirements for any in-game Dex (National/Region qualifiers) do not require any type of event Pokemon whatsoever. The slight exception to that is Rotom, who is now obtainable in-game (X/Y) through a VERY annoying way; or you can be creative and put up a decent enough monster on the GTS (I did it the in-game method but I saw my friend put up a Dragalge and he had a Rotom within about thirty minutes on GTS). Like I said, if you want it bad enough there's plenty of ways to complete the dex without going through the usual tedious process of going game-by-game and catching them yourself. The GTS has revolutionized this aspect and it's easier than ever; it'll still take some time to do it but then again if you're not willing to do that...Professor Oak would say you don't deserve the completion.
It's a bitch, to be sure; but with each generation it gets easier to obtain the more rare Pokemon, with the double edge that there are more of them to obtain. It's really tailored to the people that are absolutely insane and addicted and have already been doing this for a while. I was just lucky that I didn't sell a few games that covered the generations of what I needed. Even then, after over 200 hours in X/Y (yes, I have both; 150 hours in X, 50 in Y), I still haven't completed my National Dex; if you're looking for a comparison of how much hell you'll have to go through to actually do this. Do not rely on other trainers too much because most of them out there are assholes and they'll either screw you over or taunt you. But using the GTS wisely is imperative in terms of how long it takes to complete it. That is to say, if you're doing the National Dex. If you're doing regions well...that's child's play.