It's probably because they are SL/SL, but the Felhunter does have its perks, even as Demo, but mostly for Master Demonologist and Spell Lock - not so much anything else. SL/SL is a drain tanking build, as well. If you're Felguard demo, just stick with the Felguard; I use the imp as affliction when I'm farming primals myself, or sac a voidwalker when I'm DS/SnF.
Also, if you're on a PvP server, whether destruction or affliction, the Felhunter may still be a good choice, and the Felguard still has its perks in PvP encounters as well.