I am not seeing the point of this. If you want to search a specific forum to see if similar content has already been posted or if that forum has the answer you are looking for then it will now take 2 clicks instead of one and most likely you will need to try 2 times because it makes less sense now.
It makes sense when you are looking at a specific content type to just search that.
It makes sense that when you are looking at a category overview, to search just that.
Look at Google which is the king of Search: When you are looking at Google News, your search results will not go back to Google main search (web).
Granted that if you are looking at an Item then you are likely to have found what you are looking for and a new search may not relate to the same category. There I can see the logic.
Why not add a setting so that admins can decide themselves? At least in XF Enhanced Search. Otherwise we will need yet another addon to improve search.