If every comment is allowed it will be heavily spammed (especially if it's DoFollow). So it won't be useful for SEO at all.
Mate, would you believe what we're talking here for once? No moderated DoFollow blog with good PR? There is simply no such thing Best regards, zexy
Well, the blog's web page's itself "could" have a decent\high Pagerank value, but all the outbound links wouldn't recieve crap from it since they are diluted between all the other links.
if you want to decrease the chance of having your comment deleted, then post useful comment on relevant blogs. all blog owners need comments on their blog to add fresh content and make the blog look more active in SE's eyes. as long as you are not comment spamming and adding value to the blog article, and using anchor text that is related to the blog itself where you are commenting on, then your comment will not get deleted much. if you have a real estate related site and you post comment on a food recipe site saying "hi, nice recipe. thanks" with anchor text "Totonto Real Estate" then sure it would look spammy and might get deleted