Colm Meaney probably only got his "big role" because he was already a semi-established actor before Star Trek. I thought I even heard some where on the road that they had considered him for a main role before the series launched, but he just happened to be around for the pilot shooting and kind of stuck with it when fans started to ask about him more and more.
Although I don't doubt union rules may have contributed somewhere a long the line, we've seen plenty of background characters only ever meant to be background characters evolve beyond that role as a background character.
It seems like Ensign Gates may have been one of those characters that was in far more then her share as extra roles and was one of the few (if not the only one) that was in as many episodes as she was without having the character expanded upon, sadly, not even in fan fiction or other related media.
It just kind of struck me as odd considering almost every other TNG background character seemed to have gotten their share of the lime light (even if it was just to get assimilated in First Contact!).