That's 'cause I'm a lazy administrator
The boss used to be the sysadmin before I came on board. There's no way he was going to let me limit his local privileges to user or power user.
I also ran into too many problems with poorly written applications that threw up without explicit but unidentified folder permissions. It was easier to just make the domain user a local admin for the sake of productivity.
I know, it's not a best practice and probably very easy to remedy but solved the problem in the short run. It has bitten me several times as users have installed crap that they should not have been able to do.