It all comes down to whether or not a free stopover will still come with a roundtrip award ticket.
I believe that with AA, if you fly, for example, JFK-LHR-(anywhere else in europe you can't get to directly), you cannot get a free stopover at LHR. You need to book two awards, one JFK-LHR and the other an intra-europe award. Then you can book a one-way back to JFK. Overall, it costs more miles to have that one stopover en route.
If UA does this, it's going to be a detractor, not an enhancement, to MP.
I really like the free stopover you get with a roundtrip award ticket. I believe many others do as well.
Thus, if it means paying an extra premium for a 1-way award to keep the free stopover on a roundtrip award, then I'm all for that.