Starwood Preferred Guest has more than double the number of partners, and Diners Club Rewards (USA) is comparable (if you already hold the card). The only meaningful advantage that Membership Rewards retains is a number of instantaneous transfer partners.
IMHO, that's a large advantage that I'm factoring in. For aspirational awards, which is probably the focus of MR given the relative lack of domestic air and hotel partners (unless you consider DL, HH and SPG reasonable options), instant transfer can be the difference between getting the seat and not - especially if you're looking for more than 1 seat.
For instance, I booked 3 in J on AF (via DL) over President's Day week. At peak time like that, had I waited the options would have been gone. I consider that to be a large advantage.
I do wish that SPG could get their window down to 48 hours or so, within the timeframe that many airlines will let you hold an award. I wonder if that's part of an agreement with Amex, one that also keeps MR to SPG transfers at a bad rate.