3 ways take a lot more in the install then 2 ways. also, the K2P's are designed to be installed off axis if i remember correctly. the inverted tweeters take a lot of EQ work to tame, but sound very nice if you know what you are doing.
^What Alter AND Brandon said But with some extra notes:
To build on what Brandon said, We've had great success with the K2Ps by either mounting the tweeter low, or by kicking it slightly off axis to the listening positions. The Inverted Dome does not spread sound in the same manner as a ring radiator or dome tweeter. We've found that sitting in the "Sweet Spot" can really fatigue your ear and sound harsh.
Heard the Morels in a customers car last week and loved 'em, but not a match for the three way K2Ps.
To answer your last question, three way designs more efficiently route frequencies to a speaker designed to reproduce them. They operate under the same idea that you wouldn't expect subbass from a 4 inch speaker, just on a broader range of frequencies and a narrower spread of speaker sizes.