As noted in Blacklight's answer it's not recommended, but you can do it and everything will work, just not stellar. Here's explanations on why this might not be the best to do:
First, it makes total sense time capsule wouldn't back up files stored directly on it. Where would it back them up, to itself, the same drive? That's not a backup of much value, so this isn't a real 'negative'.
I think the big issue is it's using a shared physical disk, with one potentialy greedy process--time machine. It has nothing to do at all with the network / wifi performance.
As you're trying to shared the same disk, if time machine has a lot of unpredictable reading/writing I/O going on in the background, trying to access other files on the same physical disk as time machine could result a sporadic performance hits. If time machine goes slow, no big deal, the user doesn't 'see' it. But if it takes forever to download a large movie file to stream because a heavy time machine write is happening at the same time, the user would think the performance of the time capsule was to blame.
Apple doesn't want you to think it's products suck, so they recommend you use them to get best perceived performance.
I think if you accept the possibility of slow access for stored stuff due to time machine writing to the same disk, it should work fine (outside less space for time machine image).
However, if you're expecting blazing and consistent access to content--don't use the same disk you're using time machine for anything else. Not even if you partition it. Get a separate Drive and plug it into the time capsule an use that. The second disk will not be affected by time machines backup reading and writing.