The reason we don't keep plants or weeds, is because other than mangroves, thats what they all become: WEEDS. Like Allan said, they will eventually overgrow expensive corals and they are really difficult to control/remove. Furthermore, they are often prone to sexual reproduction, which causes the "plant" going sexual to die and release all the nutrients in its tissues to be released into the tank in one fowl sleep. Another down side of algae is that they release yellowing compounds into the water, reducing the light penetration to the detriment of corals. The only "plant" (algae" I believe is useful is an algae called chaetomorpha, which like Mekaeel said is usually kept in your sump to reduce nutrients.