WSS has a neat little feature that allows you to override the default site provisioning behavior as defined in the onet.xml file. When the WSS runtime detects that you have a provision class associated with a template, it ignores the provisioning definition in the onet.xml file and calls your custom provision class. This opens many possibilities for provisioning site in any way that you see fit. If you wanted to replace the default onet.xml provisioning, that is possible. Or if you want to apply another web template then provision files, list items, etc., no problem as well.