Hey... do you remember that one time no one else wanted gear swap but you really did?Casters already can switch out gear during combat and you don't see them doing it, do you?
I'm honestly curious how you think this is in any way different from other MMOs' gear systems. A player is always "gimp" if they don't have the best gear for the situation and its even more obvious how much they are lacking when there is one set of best-in-slot gear.
Gear changes would make more gear desired by more players because even though it might not be bis, it could be the best parrying body or has the most fire resistance (many more possibilities).
For example, as blm you can survive hellfire from Ifrit with one nail down but you will have very little hp after. If you could change instantly (Not go all blinky for 30 seconds, many games change gear without a 5 second character disappearance per equipment, I'm sure they could do it the same in 2.0) to a set of gear that would give you fire resistance or an hp set of gear, the player wouldn't have to sacrifice their damage for the entire encounter simply to guarantee the survival of one move.
Again, you already can change gear in combat but you are locked out of changing your class, you can however change your weapon to something that your current class can use. Ex. would be changing from Ifrit's bow to Moogle bow.
I'm actually pro limiting the ease of class changes to something like a mog house or at least only outside of instances but that's besides the point.
I think my biggest concern is that there will always be one best-in-slot gear set at any one time in the games existence just like any other MMO and that set of gear is almost always targeted towards maximizing your DPS. There are others things to worry about, such as enfeebling, that tend to go by the wayside when factoring in what is the best gear.
Another issue is that bis gear automatically out-dates older gear that could still be useful simply because the +str is lower. This makes older content automatically useless. FFXI did an incredible job of making older content viable and that was partly because of the possibility of gear changing.