Account dealing on bitcointalk promotes scamming, because you can buy an account with green trust and the victim would have no idea that the current owner of the account wasn't the one who earned the trust.
Account sales also encourage spam, because knuckleheads who can't write English buy accounts to get into sig campaigns.
And so on. So there are a number of us who feel strongly that it's untrustworthy behavior and leave feedback accordingly.
Although your points are correct in relation to a lot of bought accounts on this forum, they don't relate to every bought account. Some have great English skills and provide a good service. I think that we can all agree a newbie account with 1 post wouldn't be getting hired over a legendary member. Also, many people don't have the time to build up an account.
To add, the buying and selling of accounts is allowed on this forum. Even if it is looked down upon.
Nothing is ever 100%. Sure, there could be some outstanding posters who bought accounts, but I don't know who they are. That's rare. Most common is the use of a bought account by some lazy third world scunt who only wants to shitpost or scam. Can't tell who's who ahead of time, so I neg 'em all. I have removed negs in the past when someone turned out to be OK--my ratings don't mean much anyway as I'm not on DT.