Cliff, I can't see anything wrong with controlling fluorescent lighting with a photocell. But, I would be careful with selecting the photocell, to make sure that the contacts in the cell can safely make and break the Inductive currents that fluorescent lights generate.
I believe that there are photo controls out there that do not use a relay but rather a triac in the same way a dimmer does. They only go full on and full off however the waveform can be distorted like that of a dimmer and this is not suitable for fluorescent ballasts.
I think that the ballast manufacturer is just being cautious. Make sure that your photocontrol uses relay contacts and you will be OK.
I 2nd Hal's opinion. I've seen several of the photo units that "flicker" an incandescent in the twilight hours. I can only imagine the havoc that would cause with some of the cheaper imported fixtures...