Actually the question did arise a long time ago http://meta.wordpress.stackexchange.com/questions/369/are-bbpress-questions-off-topic-here :) 

I think given total experience up to now it's fair to summarize situation as following:

 - the sheer fact of plugin/theme being involved does **not** make question out of scope
 - such questions can **easily** fall out of scope by being too localized
 - given that experts in specific extensions are much less numerous than WP in general, chances of such questions getting good answers are below average (so far)