"Bulletsponge" enemies and inflated numbers don't make things more interesting.
I disagree. If enemies have more health and do more damage, it affects how you enter the battle.
If I know that the enemy is going to take twice as long to kill and is going to kill me twice as fast, I'm probably not going to charge its face as I may have in easier difficulties. It does offer more challenge. And if added properly, it can be a good middle ground between normal and (what some games call) expert mode.
That being said, a poorly implemented hard mode can cause quite a bit of frustration.