VC Fred Wilson talks about three reasons to use Disqus in a recent post:
2a) Email Replies – Disqus emails every comment to the blogger. If the blogger wants to reply to the comment, he/she simply replies to the email and it is posted as a reply (with the indent described above). This feature, which I requested the day I met/saw Disqus for the first time, is the single best thing about Disqus and has transformed my blog comments because I can now participate in them in real time throughout the day as the conversation develops. This is a BIG DEAL.