The tangible benefit to being silver is the 20% bonus (and the elite area check in is nice as well.) So let's run some numbers. Assume:

1. You can find a Fairfield for $50 to get silver this year.
2. You will stay at least 10 nights at Marriotts next year (if 11 or more, then the 11th night and beyond will give you the bonus points anyway...yes, I know that technically the bonus points don't start until the first stay after silver is earned, but I'm laying that aside for ease of calculation.)
3. Not an assumption, but a fact: for $50 you can buy 4000 MR points.

The $50 night will earn you 500 points, so you have 3500 points to go. Thus, in 10 nights, you would need to earn 350 bonus points/night to break even. That translates to a room rate of $175. Thus, if your average room rate is $175 or more for the first 10 nights, it's worth it; otherwise, it's not.