What do you think of pruning members and/or threads at your forum?

For those who are not familiar, prune means to delete members or threads based on cut-off criteria decided by administrators before. Usually it's used to automatically delete old-and-not-useful threads or to remove members with zero post count or accounts not activated yet.

I like the idea, to be honest. It keeps my forum clean. I would rather have a small but active forum than a big one but full of members with zero post or unnecessary threads. However, I just feel that it's not completely right. Like, even if I leave them alone there not much harm could be done, I could just ignore them and move on to improve the forum or whatever... I haven't been able to decide on this completely. ^^;