Both and here's why: sometimes you can't get an award flight on your preferred carrier. Because it's an award flight I will look at ALL partners for both UR and MR and pick the best option (price, route, destination etc). When you redeem points your status doesn't really matter. I'm headed around Europe for a few weeks and although I'm Delta loyal here in the US and a DM it's just about the only airline I'm not flying around Europe. Same applies for hotels for the most part. It's a free room. Sure some programs will get me a free upgrade but I'm valuing the ability to be a consumer based on price alone and ignoring my loyalty. I'm much happier picking the hotel based on location and not being stuck to choose from the one or two loyal brand hotels that most of the time are not convenient for my travels.
Open123 nailed it with SPG points. One must have some SPG points to fill the gaps in both programs, or to accentuate the mile transfers with the SPG bonus.
