I also maintain Platinum status with both SPG and Marriott and I like the situation just fine, mostly because of my destinations, in larger cities I use SPG and when I am stuck in Tupelo, MS for 6 weeks I stay at Marriott properties.

It's all about what you like the only tangible benefit I can think of for going 100% MR is Lifetime status which takes 12 years at minimum to achieve.

Until something drastically changes I will keep doing 50 nights per year with SPG and around 100 with MR, if nothing else it gives you more options and lets you jump back and forth depending on who is running the best promo that month.

No real downside in my book to keeping your options open in case a good room rate, hotel location, or promo comes along.

Will be watching this thread in case I am missing something as well.