As a previous player of FFXI, MMO's, DnD, and all other things considered to be of this genre, I can safely say that it would be perfectly reasonable to change something in the battlefield. Would it be wise to change out your suit of armor within battle? Not likely, but if you had a reason to do so, then go for it.

In FFXI I used to constantly change out wands, instruments, earrings, etc because it suited the situation better. I think anyone that ever played a bard in FFXI knows of these circumstances more than anyone else.

That being said, I do think some items (like wearable body gear) should face a mild penalty in doing that action. Something along the lines of a slight stamina hit, or like 100 TP to do such an action. This way it would make you think a little more about switching gear than a free for all system.

If people find gear swapping ridiculous, gear changing could be restricted to passive mode, but I would think that there will still be a way to create an edge... How about if you have the opportunity to train at dojos related to each of the elemental affinities. Once you have completed your training at that place, you would have a new battle stance. For instance after training at the Earth dojo, you would have a better defensive stance, allowing you to absorb more damage, but would cause you to deal less damage. Or completion of the Fire dojo that would allow you to hit for more damage, but you would become squishy. Wind - Attack faster, higher evasion, but lower damage. Similar things could be set up for the remaining elements.

Customization is a key for any MMO, and if that includes swapping gear to better optimize yourself than how is that any different than socketing gear, magical enhancements, food buffs, or searching for that ultra rare armor?