I find nice benefits in a low-light system when the tank is just setup. The plants use the CO2 and grow more efficiently, especially since most people load up on nutrient-rich substrates. If ammonia is kept down by good maintenance, then this type of setup can experiences little algae.
Once a tank ages, however, I, personally, see no point and noticed no marked difference in growth, and I often then remove the CO2 injection after that. By then the tank sort of takes care of itself if you do waterchanges. This has only been my experience, however, I'm not sure if others have had this as well. I think this may be due to the nutrient-rich portion of the substrate kind of losing its punch and the plants grow slower and enter into a long haul. We're still talking healthy growth here, no defficiencies or anything like that. At this point, I tend to reduce the number of stemplants in my scapes in favor of slower-growing root-feeders. With my 20g low-light, I stopped using stems about a 15 months after the tank was initially setup, which is rather a long time to use stems. With a high-light tank, the stems seem to be spent a lot faster, sometimes in as little as 3 months. At ths point with my tank, everything just kind of stops and growth is slow, evident, but slow and the tank enters a time of great stability. You can maintain a scape for years like this with little effort.
This is only my opinion, though.
llj
