Bio-balls will help with filtration, but you must maintain them as with any other mechanical type (sorta a biological type too, in essence) of filtration.
Most wet/dry sumps sold at the lfs
afford room for a skimmer.
Many around here would say the same setup, with the bio-balls removed and live rock in their place are a better solution.
With proper maintenance, the bio-balls are an acceptable solution.
Any way you slice n dice it, the sump (and that's what a bio-ball setup is) has to contain some measure of mechanical/biological filtration, be it bio-balls and a couple of grates and foam, or a series of baffles and live rock and some foam (or mesh, or bags of this n that, or both and there's always something else someone comes up with!).
There's lots of arguements any which way.. lots of pros and cons for all of the above.
Figure out what works for your bank-roll and maintenance ability and stick with it.
Consistency in maints is really the biggest thing.