Mangroves (in large numbers for aquarium purposes) are known to remove nitrates; I don't believe they have any sort of effect on PO4.
Regardless, they do (as you say) look neat. Be warned; they export salt through their leaves and require frequent spraying of freshwater to prevent them from dropping the leaves and eventually dying.
Doing water changes, running phosban or ferric oxide, gravel vacuuming and a good protein skimmer are much more efficient ways of controlling nutrient levels. JMO
his article may be of interest as to some of you as a starting point:
http://www.advancedaquarist.com/2002/4/aafeature