Fundamentally, it's their flexibility that makes them valuable and the SPG/Marriott merger has increased that flexibility further. Amex MR Points transfer to SPG Starpoints at a rate of 2:1 (so you need to transfer 2 MR Points to get 1 Starpoint), which means they also now transfer to Marriott Rewards at an effective rate of 1:1.5 (1 Amex MR Point gets you 1.5 Marriott Rewards Points). You just need to transfer to SPG first and then across to Marriott from there. This is a big deal, because until now it's been difficult to acquire Marriott Rewards Points in significant quantities from the UK unless you stay a lot at Marriott hotels – and that opens up a huge range of new hotel redemption options (even pre-merger, Marriott had the best part of 5,000 hotels!). It also means that Amex MR Points now transfer (directly or indirectly) to Hilton, Club Carlson, IHG (via Virgin Flying Club), SPG and Marriott, at varying rates of generosity.