Interfaces are an in interesting topic to be sure. The advantage that "bigger" languages have is that they can enforce the interface - in VB using the implements statement. That keeps intellisense working and helps the compiler do the checking. SQL doesn't have implements, so you have to enforce it by convention.
From a client coding perspective it makes sense to do this especially if you're manually populating the parameters collection in ADO to avoide the extra round trip. You can change the proc name and everything else works.
I'd be interested to hear your response about why it wouldnt be just as effective to put all the rules in one proc and just branch to the appropariate code based on a passed parameter.
Andy