For a widely popular software like wordpress, security will be a cat and mouse game - until it reaches a level of maturity like other open source projects (ie, linux).
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.