As this article about Frequent Flier accounts points out, there are rules about account dormancy that need to be followed (even in Marriott Rewards) and more importantly, that the vendor reserves the right to alter or terminate the program at any time.
Here is the Marriott Rewards T and C language:
"Additionally, the Company has the right to terminate the Rewards Program by providing written notice to its Members six months in advance of Rewards Program termination. In that event, the right to earn Points and Miles and redeem Rewards may end six months after notification, no matter the extent of Member participation in the Rewards Program. The Company may terminate the Rewards Program earlier in whole or in part in any jurisdiction(s) if required to do so by applicable law."
That's one reason I try to stick with one airline and one hotel chain as much as I can. In addition to consolidating points/miles, I usually don't have to worry about expiration. The point about changing the T&C at any time is one of the reasons I try not to hoard points as well. I'll usually save up enough for a big vacation but not much more. It seems like hotel programs are a little safer, but with airline bankruptcies/mergers, you never know when your miles may become worthless (or somewhat reduced).