IMHO, Marriott is the best at offering incentives to keep staying once you hit the highest tier out of any of the other programs. All of these have been mentioned above:

(1) Possibility to hit Plat Prem
(2) Accumulation to lifetime status
(3) Rollover nights

Personally, I hit Plat (again) in Feb and am still pushing all my stays to Marriott. I am still Diamond HH and Plat SPG but will drop those after this year. JMHO.