I would think the larger concern is not with the small group of loud complainers that don't understand the concept of testing. I think the concern would be that it could have a negative impact on future potential buyers.
I don't know anything about how Steam forums are managed, what gets carried over, and what controls you have over what reviews are visible or remain after the game moves out of "early access". Whether we like it or not, there are many potential buyers who, instead of looking carefully at reviews, will look at overall recommendation ratings on steam as well as metacritic scores. The problem is, if the reviews carry over to final release, that would be a problem. Even if the forums would eventually have those alpha comments buried, if the reviews can stick, I would be concerned.
We saw something similar happen with Elemental. Even if after a couple iterations, the game became a good game... it lost all its steam because of the reviews stuck. (granted, that was for a game release, not a testing version).
Whether the game is sold through steam now or not, I don't have a strong opinion, as long as you are able to distinguish this version from the release version from within Steam's architecture so as to minimize the impact of the current batch of steam trolls.