Comment 12 for bug 490983

That is a bit of a legacy process, and looking back I think it was decided to install both as a convenience should anything require it from stock packages. That said, if anything is going to be compiled with mysql51 installed you'd want to, and would be compiling against MySQL 5.1 and not mysqlclient15-devel header files.

If you could report a separate bug report regarding 'removing unnecessary dependency of mysqlclient15-devel' I will look into removing and testing it out. You can also submit a merge request:

See http://wiki.iuscommunity.org/Doc/DeveloperGuide#Contributing_Packages_or_Changes