I really love this simple plugin -- it does the job in most of the cases.
However, there are still a few spam registrations that slip through.
I would recommend taking a look at two of the actions that SABRE takes to further limit these registrations:
- Unobstrusive tests to detect if registration is done by humans or not (e.g., too speedy?)
These would seem to go naturally with your approach.
You might be wondering why I don't use SABRE instead -- for some reason, I encounter too many bugs with it.