Very much agreed - Resolving an encounter, should reward XP, otherwise players are going to feel pushed towards violent answers all the time.
It would nee dot be managed as an individually isolated thing, however - an encounter is blocked to contain a certain number of hostiles if triggered into violence, and pays, between them, a certain amount of XP; that's as normal... but you'd need them to have internal flags as to wther they have "paid up" xp already at any point, so players don't feel tempted to 'double dip' with a peaceful resolution, followed by a murderous heel-turn, just for double XP.
Fortunately, Larian already have this in place, or a working version of it, in their one-time-XP only knock-out mechanic... so there's seemingly no reason for them NOT to be awarding XP for social or clever resolutions.
(As an aside, this doesn't go into the manifold ways in which their current knock-out system is ridiculous and badly designed...)
