I thought this was odd also. The only motivation I could think of, is that Limsa currently only has one active DoW/M guild (since musketeers and arcanists aren't active yet). If GC gear ~ city guilds, Grid and Ul'dah would have gear for three classes, while Limsa would only have one.
In response to OP :
currently every quest in the Grand Companies is mirrored between the three. I fully expect that this will continue.
I expect the gear from GCs will continue to be equivalent stat-wise to good crafted gear, so no one will be losing effectiveness by choosing a GC. GC Gear is intended (I believe) to help give the nations a sense of identity within the players' minds.