Hi Steve
I do not question the use of blog comments at all, in fact I think it is a good and easy way to build links.
I just do not see what an extra service could add as all steps can or even must be done seperately from such a service, too.
With my search it is no problem to come up with hundreds of blogs in a short time and commenting and moderating still has to be done manually.
Closing the comments is a good idea but can be done with free plugin (e.g.
http://wordpress.org/extend/plugins/comment-timeout/), so the only advantage would be that you would have a slightly higher chance of approval.
But again, I would not let people comment unmoderated even if they paid for a service as there is the temptation for them to write only short unrelated stuff to have less work (after all why work so much, hey I paid...)
Mike