I'd rather it be mercenary ships than a third faction but some people apparently feel like sharing ships would somehow destroy the faction system... I don't buy it.
Different people in this thread are talking about "sharing ships" in very different ways.
Some use the term (or a similar one) to mean having ship/pilot combinations that can be joined to both imp and rebel squads
Some mean having ship/pilots that can join neutral fleets or one of the other factions
Some think it means having the same pilot, in the same ship, but with different pilot ability in different factions
And some are talking about just the ships. Having the YT-1300 in the fringe faction, but with completely different pilots
And really, the whole concept has been so thoroughly tainted by attack wing doing it as badly as possible that most people can't separate the basic concept from wiz kids atrocious implementation.