Plants will help, period. Specifically with keeping water quality up.
The ideal setup for each may be different, but that does not mean that it is not a good idea to put plants in a discus tank. You may not want to use certain supplements, but this simply means your plants will not do as good as they possibly could, not that they won't do very well in with the discus. If you use hardy, low light plants they will do fine.
Discus are from the Amazon River. This is not a river devoid of fish except for discus. There are MANY species of fish and plants in the same exact waters as discus. There are many species than can be kept with them and both will thrive.