In all the games I've used him, a number beyond reckoning, he's only failed his test a maximum of three times. And the worst effect of that was losing a Galadhrim Warrior to his outburst of insanity (curse you Southy...
).
Broken Mind really isn't as bad as people think it is. A relatively high courage and decent amount of will points means that he'll rarely fail it.
And as others have said, Denethor is one of the key leaders for a horde Minas Tirith army. If memory serves I had around 75 models on the board for an 1000pt game using Gondor... that rivals a goblin horde!
Sure, he isn't an amazing army-destroying hero like his son. But he brings more numbers to the field and isn't too bad a combatant if you use him well. Don't overlook the mad king