Rotala is actually a moderate to high light plant. Mine does really well in my tank with high light and co2. I tried putting some clippings in my low light / low tech tank and it didn't do so well. However, as you mentioned, the others such as crypts, java fern, and various mosses will tolerate low light conditions. Swords would be okay too as long you can supply the roots a good source of iron (i.e. Root tabs).
Anyway, to the OP, i would think the only disadvantage, IMO, is if you don't take precautions to clean and sanitize a plant before introducing it to your tank. I've accidentally introduced nasty little critters and parasites by just throwing new plants in.
Overall, the pros of live plants are just too many to pass up.