The point is that you find the MP level that's most efficient for you. Running MP10 is going to be too slow for most people, so you choose the highest level you can run reasonably quickly.
Better gear lets you fight harder monsters, and do so at a higher speed and with less risk. That's kind of the point. What else should gear do?
How should it all be balanced so that MP10 isn't "mandatory", and yet still worth doing? The problem is that so many people think that the best option is always mandatory, no matter the gap between best and second best. If best is 1% better than second best, you'll still have people complaining that there's no choice in the matter, and they're "forced" to play a certain way.
And yet somehow there's all these people complaining "don't tell me how to play!", whenever Blizzard reduces the gap between best and everything else, that Blizzard is "taking away diversity", when actually, they're increasing it.
People need to learn that "viable" doesn't mean "best".