Actually, I've had Wyll not leave during the goblin raid - as long as he's not present when you make your plans with Minthara, and doesn't see you (for example) shove Zevlor off a cliff to kick things off, he just goes along with you and participates in the slaughter just fine.
This may be a loophole in Larian's sketchy coding though, since I think his party camp dialogue was the same - the generic "here's to heroes" dialogue.
