No follow, no fair!
If I am discussing a point raised in another blog, I will try and be a good netizen and set up a trackback. These are by no means automatic for a Blogger.com user and require you to make the effort to perform a manual ping from the Haloscan website.
If blogs do indeed drive 'global conversation', then a blogger should use the tools available to them, such as enabling comments, trackbacks and maintaining a blogroll.
I have also noticed that depending on the calibre of the site you are linking to, trackbacks provide reasonable levels of quality visitors who generally contribute more than the average search engine visitor.
However, on a recent link popularity check of my inbound links, I was surprised to see that the trackbacks do not appear, even though the direct link to my blog appears in the source code.
A quick search of the source code on some of my blogroll revealed that the majority of trackbacks (including those on this site) contain a 'nofollow' tag which will prevent the search engines from following that link. Clearly, this is done to prevent people abusing the system to obtain 'free' links from high PageRank websites.
Trackbacks are a diluted form of link exchange and the 'nofollow' barrier doesn't exactly encourage you to link to others. The problem is that setting up a trackback whitelist would be more trouble than it's worth and the hassle of comment spam is worse than the joys of interlinking.
Perhaps it is better to give than to receive.