I'm already really impressed with how polished the first act is, and there's only been one patch. So if Larian got the first act so good on the first go, I'm not as afraid the later acts will be weaker without an open alpha all the way through.
I think they're most interested in everyone's impressions on the overall systems and flow of gameplay and tone of the game, most of which we can see in act 1, not as much if such and such quest is good. Specific feedback and bug hunting is good, of course, but that's more in the domain of professional testers, imo. Mass testing of late game balance is probably the biggest loss, but hopefully Larian learned their lesson from D:OS 1.
It's just probably not in their economic interest to release a full alpha. Many people can't resist the temptation to play through the alpha, but then won't want to play again on full release. It's best to get as many players on release as possible, which boosts sales. A full alpha takes some oomph out of the official release.
