I posted a comment on the blog which hasn't shown up yet. I'll recreate it here.
Trying to recreate cron in Java/Ruby is asking for trouble. Any real-world application will have activities which are hard or impossible to implement in Java or Ruby. They will be OS-specific, or low-level, or be needed to monitor the very container you are trying to run them from. They often need to be extremely robust (more robust than Java apps can be) in the face of very unusual conditions on the box (huge load, almost zero memory available, etc). Cron and OS-specific tools are well-suited to these situations. Java and Ruby are not.
I'm a software engineer who has been working with Java since 1.0 beta, and I find it a great development language (I'm less experienced with Ruby but it is nice also). But they are simply inappropriate replacements for cron.
I understand the desire to stay in the productive, known world of Java or Ruby. And for periodic tasks "within" the application, some sort of cron-like facility is a nice feature. But robust software systems that are resilient and handle 'unexpected' (i.e. difficult to handle) situations without simply falling over require developers to step outside the virtual machine and deal with the OS and hardware (and databases and filesystems and other software systems) that the virtual machine depends upon. For those watchdog or maintenance activities that have to occur outside the virtual machine, cron (or it's OS-specific equivalent) are indespensible.
If you really think you can build a non-trivial system without cron, you've failed to identify or are ignoring requirements which you must satisfy to make your system robust.