Patrick Beverly subs in for defense--But why not Ronnie Brewer also?

Im a big supporter of the fact PBev gets brought in for Jlin during critical Defensive situations near end of games.
(Love Lins effort on D but PBev got slighty quicker feet on D)

But why is it that our worst defender - Harden - stays in?
when you have Ronnie Brewer who's a much better defender with great wingspan.

yes harden is a better FT shooter, but vs raptors we didnt have a situation on D that warranted a foul situation we simply needed D...and Ronnie would of given us some much needed stops.