Why It Sucks When Other People Do It: You have to beat everyone in this room before you're allowed to leave. The first 49 minions of the Dark Bad Power Guy were slain without too much effort, but then this asshole shows up. And blocks. And blocks. Sometime he doesn't NOT block unless you hit him in the back. Too bad he never turns around.
A gimmick like this can turn what should be a simple enemy into a nigh-insurmountable challenge. Not that there's anything wrong with that, but when you have 6 nigh-insurmountable challenges surrounding you, all of a sudden Easy Mode starts looking pretty prime.
Why YOU Have To Do It: Part of the fun of platformers is your remarkable prowess. You can take out a room full of enemies if you do it right.
Problem is, just because you can kick copious amounts of ass doesn't mean you can put up with a lot of damage. Hitting a common opponent a dozen times isn't that odd at all, but getting hit a dozen times is always bad news. If you don't block, you die. Plain and simple.