^Well, Andre Braugher is one of the leads on the TNT series Men of a Certain Age, so it wouldn't have been practical for him to appear on a more frequent basis on House. Of course, they could've cast someone more available in the role of Nolan, but several of House's executive producers have worked with Braugher before (David Shore on Hack, Paul Attanasio on Homicide and Gideon's Crossing, Katie Jacobs also on Crossing), so I can understand why they'd want to work with him again and why they'd want to cast a really strong player as House's therapist.
I guess, but I don't see any reason why they couldn't have cast someone else in the role who would have been more available. Not to knock Braugher's performance at all, but I think having House's therapy sessions as a regular recurrence (though not as a framed story as we got this time around, as that would have gotten tiresome fast) would have been a really great direction to take the show this season. I'd rather have seen someone else in the role as a regular fixture rather then getting Braugher bookending the season.
Ah well, it's sort of pointless to harp on it, what's done is done!