It is constantly argued that vanilla and TBC servers would be a bad for the welfare of World of Warcraft; however, there is one major consideration and positive effect that I have not seen a single person address.
While I am in favor of vanilla and TBC servers I am going to be completely honest with myself that I may have a case of rose colored glasses syndrome; then again I may not have rose colored glasses syndrome are parts of the game in the past may actually be better. But how do we sort this out? How do we tell between the passing of time distorting a persons view on something and that their view is not distorted and circumstances were actually better in the past?
The answer is classic servers. I do agree classic servers come with drawbacks, but the most invaluable thing that classic servers contribute is increased feedback.
Blizzard constructs its game based off of player feedback, and if players are able to simultaneously contrast and compare difference between classical wow and modern wow in real time, then blizz can use this massively increased well of feedback to understand what players like on a much deeper level. Players would be able to give feedback about what they like to blizzard spanning multiple expansions.
You cannot put a price on feedback.
TLDR: Basically if classical servers were made possible blizzard could pool feedback of what things players enjoy from all expansions at once without worrying about time distorting players views. This would create an invaluable deeper well of feedback that, potentially, could massively improve the quality of future expansions.