We have taken over support a few new customers recently and they all have had a third party backup software installed on their Small Business Servers. I have no idea why other IT companies do this!
SBS comes with a built in backup manager which is perfectly fine for most situations. It also works well with external hard drives, managing the rotation of disks and providing a simple interface on the status of failed or completed backups. We try to keep all our SBS boxes a free from 3rd party software as possible, this makes them run faster and with much higher reliability.
So my advice is, if you own SBS2003, don't waste your money on 3rd party backup software, it will cause more problems than it fixes!