Is There a Reason Not to Use the System Stored Procedures?

This is directly related to my other question about roles and the membership column.  Rather than write my own stored procedure, as the thread title states, is there a reason not to use the system stored procedures to add someone to a role or even add a new user?

[EDIT] I got myself a little confused based on the fact that there are stored procedures dealing with articles and such.  The sprocs I was looking at all have to do with SQL server and not Sitefinity from what I can see.