For me the problem is not that they release frequent updates - I would have a problem if they didn't release frequent updates to known severe vulnerabilities. If you look at their release logs you will see that most of their security updates has 24-48 hours of turnaround from the time of the vulnerability detection.
The recent vulnerability that this blog and most others (ie, smashing magazine) got affected has been fixed 2 releases before the current release. People who are affected didn't update for the last 3 release and someone developed a script out there that automatically scans for that vuln. and takes advantage of it.
From a security standpoint Wordpress is a trivial application because it's almost entirely read-only, except for the comments. Making a plugin-architecture bullet proof is a different story, but as I understand it most of these issues affect the very core, i.e. a vanilla wordpress install.
There is no excuse for their sorry state of affairs.
Why do there have to be frequent security updates in first place?
If they didn't manage to tighten up their very limited attack surface in 6 years then I'd say it's a lost cause.