I don't think it did us much harm replacing Quincy with Vinny on a scale based purely on ability. The two are pretty equal in that regard -- prone to mistakes, capable of making some big plays.
I think the damage done in that move, in hindsight, was the message it sent. Everyone knew Vinny was brought here to provide insurance, and to possibly challenge for the starting job. Throughout mini camps and the first few days of training camp though, no unbiased observers indicated Testaverde looked better. Quincy wasn't setting the world on fire, but you could tell he was the team's starting quarterback.
Then he's outright cut. Granted, it was probably a move that needed to be made, but it set this team back mentally. The players knew we were suddenly forced to start a player that wasn't previously expected to start. It's a shame that Quincy's irresponsibility cost this team in such a manner, because like I said, I don't really blame Parcells for making the move. He was just playing with the cards dealt to him.