This is definitely what I dislike most about the app. Other than not being able to browse the forums this app is terrific. It would also be nice if you could refresh/update certain crags not the whole area.
The MP app isn't meant to be a replication of the entire site in a stand-alone app; it's a handy routes database that you can use anywhere, even (or especially) when you can't get to the internet. If you have no internet access, the forums would be unusable, and if you do have access, the full website (including the forums) is available to you from your iPhone. So having the forums on the app would be pretty meaningless.