I agree with herefishy that co2 is an extremely helpful tool in preventing algae growth. If you have a high light tank, your plants will also require a lot of fertilizer and co2. It's easy to add more ferts, but it can be rather difficult to stock the tank to produce enough co2.
For instance, I have a 10G heavily planted tank with a 40 watt compact florescent light. I'm not even sure it would be possible to fit enough fish in there to provide the co2 required by this many fast growing, high light plants.
Also, I'm not interested in adding more fish. The tank currently houses a single C. Lorteti and that's it. Before he was in my possession he terrorized a myriad of bottom dwellers, a small PIKE and also killed his two female tankmates (in a 30 gallon heavily decorated). Clearly this fish cannot live with other fish in a size tank I am prepared to provide for him. While yes, the tank is not at full bioload capacity, I would consider it fully and properly stocked, as territoriality must be taken into account.
I also agree that a fully automated system would be overkill. I highly recommend using DIY co2, as it is cheaper and won't be capable of producing a crazy excess of co2.