This. A thousand times this.
The battle system may not have been designed to offer gearswapping as an advantage, but you know what some players are like and doing things that 'weren't designed to happen' (Exploits, bugs etc).
There are some that will use this regardless of how the system was designed. Let's just make it so you can't change while in battle and everything's gravy D:. Even if they set it to 'no gearswapping unless passive' I would be happy... ><
Edit: a 3 second stun/penalty effect would be cool too e_e haha if someone tried to swap while they had enmity or if the party was in battle. It'd allow for people to re-equip items if there are bosses that unequip things, but it'd stop people from abusing it to min/max stuff on a whim like it was in XI