As long as it doesn't actually break anything, then I'd call it a legitimate method of making a hard mode. Most of the time it just means there's less room for error, or that you actually need to make full use of the tools provided.
Ah, but that's the thing, the game doesn't require you to be more skilled, it simply punishes you more for your mistake, it does nothing but reduce the options the player has, which can easily make the game uninteresting.
Let's compare Doom and Doom 3. Doom has awesome difficulty levels, adding more enemies instead of everything just doing more damage. The gameplay is the same, guys take the same number of shots but you have to do it more often to succed. You need more strafing, more aiming and more crowd control. Good stuff.
Doom 3 had terrible difficulty levels. Instead of facing more zombies, each one took 4 or 5 pistol shots to die, which forced the player to walk backwards and shoot over and over. Not fun at all, the difficulty forced players to play in a boring manner.
Proper difficulty levels require very little effort for the amount of joy it gives to the audience. But when developers do not even care about proper customizable controls...yeah.