Well there is a lot of debate over issue i guess. There are those who don't feed their SPS because they don't feel the need to, or they are concerned that they may overfeed their tank. The added "pollution" wouldn't be so great for the SPS that like nothing but the best water conditions.
On the other hand there are those who have tried it, and will swear by its benefits. They may have a system with a pretty serious skimmer. They are usually worried that it is removing a lot of potential food and decide to supplement with something like DT's.
I think its totally up to you and what your system is like. It doesn't hurt to try it out and see what the results are.
Here's a little reading anyways:
http://www.reefs.org/library/talklog...hi_050398.html
http://www.reefs.org/library/talklog...en_102500.html
http://www.reefcentral.com/modules.p...e&pid=5&page=2
http://www.dtplankton.com/phytoplankton/benefits.html