well - you need the land of demons for the main quest - as the key that opens the last path (in elven lands) was stolen by orcs, but they opened demon gate and lost both keys (demons got them and protect them... the main quest key is well protected in demon castle)
Hint - when going to demon lands - don't count with red or black dragons in your army - all uints in demon lands have high fire resitance - any other units are better
(as non of demons can really fly, the giants are just great
)
And yes - you can use demon units in the last fights (when you will fight mostly dragons only) when you combine the demon units with the correct artefacts - your resistance to fire will be 70% or more, so closing the gate of hell is very questionable regarding this facts. (Demon and Arch Devils can really kick dragon asses