It does not hurt any nodes used for the connection's anonymity, it does hurt their connectivity and performance. Tor nodes are publicly known and are not the same as Tor users. One of the many problems of BitTorrent or any high bandwidth application over Tor is the lack of capacity in the network (but there are others, as mentioned in my previous comment).

BitTorrent has to provide an IP address; it is part of the protocol. The issue is that some, most clients even if configured to use Tor will leak the true IP address of the user and not use a Tor exit's IP. Not good in any case.