I would like to know your opinion about the subject that we could already observe but only on a small scale. That is buying expansions that we don't need for a card or to that we want to have.
At this point he good example of situation is "Squad leader" it comes only with Tie-Advenced so any Rebel player that wants this card must either buy that ship or get one from Empire player. The problem isn't big (so far) because there are many Imp players that bought multiple copies of that ship and have spare cards (they need only one, after all). But with wave 2 the situation may get worse. I'm Imp player and if there will be a card that I like a lot that comes only with Falcon what are my options? It is highly unlikely that rebels will buy multiple copies of that ship, or maby the cart will not be unique so they will want to use several copies of it?
Do toy think that may be an issue in the future?
If you are just playing a friendly game, you could always go the proxy route.
If you are building both sides at the same time, this shouldn't be too much of a problem.
If you are only building one side, it would be good to check with people who are playing the other side and see if they are willing to let go any spare copies, eg. You can only have 1 squad leader in play anyway, so you could easily sell/trade the other copies of squad leader that you might have with others.
My only gripe right now is if i need multiple copies of that card, eg. I once toyed with the idea of having a 6 Black Squadron TIE swarm where all of them were equipped with expert handling, but I definitely do not have 6 copies of that card, and well in friendly matches it wouldn't be too much of a problem, just proxy it, but if for some reason you wanted to use it for tournaments, you will have some problems.
tl;dr, it generally shouldn't be too much of an issue if you are not playing tournaments. If you are playing tournaments, try to borrow from others. If you wish to own a copy of that card, perhaps you could get whatever set it is and sell off the rest of the stuff that you do not need. I'm sure some non-playing collector wouldn't mind having one of those miniatures at a low price, say around half of what you paid or lower