Far as I've seen none of the gear is really truly beneficial over what can be earned elsewhere, materia-bonded stuff tends to make all that gear obsolete. Also, isn't SE inserting equipment for all classes regardless of allegiance to the higher up tiers? Last I recall there was a Gridanian Shortbow and Longbow being added at 40 and 50 for the Twin Adder at higher up promotions. That makes joining Ul'dah just to get the Ul'dahn Longbow obsolete. As for content, just about everything you experience between now and 2.0 is a one-time shot anyway, so it's not like it'll matter if you created a new character just to experience that stuff or not anyway. If you don't do it now, you don't do it period.
Still, I agree there should be an option to abandon your Grand Company at a later date that is. After the hells that threaten to destroy Eorzea have passed and we're into the Seventh Umbral Era (AKA 2.0's release.) we can talk about swapping companies then. Personally I think if we are allowed to switch you lose all progress you've made in your current rank, but keep your seals. So if you're a Sergeant in the Immortal Flames, joining the Twin Adder makes you a Private Third Class again, and if you swap back, you're still a Private Third Class upon rejoining the Immortal Flames. Consider it as a means of retribution for abandoning your army.
Also, personally I kind of wish they hadn't made it so you were forced to join a Grand Company just to get yourself a Chocobo, or participate in content such as Primal battles. This forced some people who weren't positive which company they wanted to choose anyway just to experience this content. I planned to join the Twin Adder from the start (Go Gridania! Whee~!) but I know a few friends who joined a company just because they had to so they could play with the rest of us.
EDIT: Also on a slightly off-topic comment, I hope they will allow you to change home nations in the future. Unless we're given a choice where to base our personal homes at, I'd hate to be restricted to living in Ul'dah. I loved their opening storyline but I love Gridania more, let me live there please! Thanks. Anyway, on that subject, I think allowing you to change nations would work best if allowed after completing the main storyline objectives.