Basic: It's a nutrient and will promote/accelerate growth/bloom of undesirable things like algae, which can block light from the water as well as stop/starve the exchange of oxygen with the surface. Wreaks havoc with the ecosystem in that body of water.

http://bcn.boulder.co.us/basin/data/NEW/info/TP.html

All said, I get excellent results from a number of phosphate-free powders.