Nofollow seemed like a good idea when it was first designed, but it shouldn't be enabled by default in future WordPress releases, because:
- It hasn't stopped or even reduced comment spam. Akismet stats (and my own, more limited, experience) show that comment spam is more prevalent than ever before
- It is no longer necessary to deprive PageRank to comment spam that gets through, because filtering solutions in WordPress (I'm thinking of Akismet specifically, but also SK2) have improved to the extent that it is very rare for any spam comment to be approved - at worst it gets moderated (still no need for nofollow).