As it stands now, and as far as I know it's been this way since the start, switching weapons in Turn-based is a Standard action. It takes your whole turn.

I think it would be a lot better served by being a free action, but I can see how that would be problematic for balance, as it could let you swap back and forth multiple times per turn, let you always end your turn with a shield for better defense, etc. BUT...  you already have OTHER systems in the game which are free actions that CAN'T be spammed.

For example, you can't toggle a modal weapon mode on and off multiple times per turn. Chanters can't switch their chants multiple times per turn.

So what was the rationale behind this design decision for weapon switching?

Look up the Quick Switch ability + Black Jacket. It turns weapon switching into a free action. And it's kind of OP.

Because you can switch to a very offensive setup right before your attack and then switch back to the most defensive setup.

That's basically why the normal weapon switch is not a free action.

I feel spending a turn to switch weapons is a fair trade.

the point is fair - Grimoires can be swapped once per turn. Black Jacket should have the same system (?) - one free swap per turn. I see how completely free swapping might be a more direct translation in RtWP system, but I think the pacing makes the difference.

 IF THE GAME HAD action points and every ability or attack or using a potion would cost X action points , all would be perfect. Action points in that case would depend on dexterity and attack speed bonuses.