And a lot of people don't prefer the flat sound either -- it sounds a little bland and un-musical, at least to my ears. If you compare the Etymotic ER4P to the Shure SE530, you'll note how the SE530 sounds much more bass-and-mid focused, but it's in a good way. Ultimately, it's personal preference.
Obviously, though, for accurate monitoring purposes, you definitely want a flat response. I can't wait to see how well this fares against the UE-10, which I believe is the current undisputed neutrality-king.
It is like going to a really high-end 3 michelin star restaurant and throwing ketchup on everything because it tastes more "familiar/comfortable".
Or buying a Picasso and putting it behind yellow glass because you like warmer colors.
Destroying what the chef/artist/engineer wanted you to experience in the process.
I think the UM3x is for me... I love flat and neutral. I am an Audiophile and Art of audio production for me is the gear that brings me closer to the recording as it is on the media it is on. Because that is the image and information the people that made the music want me to hear.