Having a tertiary just leads to needlessly hoarding weapons, in my experience.
I tend to hoard weapons anyway. Worst is the likes of TES where you can bind anything to one of the hotkeys so I end up lugging about all sorts of useless crap that I never use.
I did think Divinity 2's three ready weapons was a bit excessive, and only mentioned it from the point of view of there being some history; but two weapons at the ready would definitely be nice.
