I totally agree, it is ultimately a short sighted link building technique. And indeed, if the amount of time put into spamming was taken to create and market a genuinely good site that is useful to people, then it is probably less work than spamming ultimately!
The thing is though that for good or bad, it ain't short sighted - and I write as one who deals with this kind of signup every day.
The cost to the new 'member' of doing the signup is zero. The time taken to do the signup is, again, zero.
The benefit is expected to be a backlink to one's pages.
So, why NOT do it.

Stopping the problem is not too hard - I solved it by taking away the ability to get a live backlink to people who were not accepted contributors.
A while back though the bloke behind one of the better known forum signup and auto backlink building tools said something like this:
"Each time a person signs up to make an account on a site, or posts and rss feed, an article or makes a blog post the Alexa rank of that site is increasing and that can be a real benefit to the sites receiving the traffic"
Now, this can be very true, I have seen it happen. I have also seen the effect of it happening and it is not just Alexa. At the least I am now able to command a higher rate for advertising on the site - because of the increased Alexa ranking, I get slightly better quality Adsense ads and hence conversions and revenues, my traffic overall has increased.
The cost to me?
A small amount of bandwidth that costs me nothing. I brought forward my plans to optimize my database and server so my site now runs faster and I closed down an unproductive section of the site.
If you ask me whether these spam signups were an issue I'd have to say that on balance they have not been.
BUT if I were to get much more of the content - articles and blog posts produced by these tools I might have a different perspective for a few days, but ultimately it'd result in better quality content on my site as I'd simply automate more of the review process and set the bar higher than it is now in order to slice off the autospun almost-English that passes for article marketing for most people who call themselves article marketers. To be honest, increasing the overall quality of the content on my site is a major goal and part of an ongoing initiative.
Oh, for the sake of disclosure, I use a tool that sets up forum profiles. We do not make spam posts to forums, or any other type of site, and I pride myself on NOT vandalising the interwebs.