Snail mail and faxing is not only archaic, it's not green. But, in an age of identity theft, it's really the best way that Marriott can legitimately credit an account and have an audit trail to back it up. In the worst case scenario, snail mail or fax protects your Marriott Rewards account.
That's why it's a Best Practice to present your Marriott Rewards ID plus credit card and any identification required at Check-In. If the Front Desk does not acknowledge that your MR ID is attached to your reservation, then it needs to be added otherwise you won't get credit in a timely manner.
Another way to make sure is to sign up/opt-in for e-Folio. That way, a statement is e-mailed at check-out or w/in a day or two. When the e-Folio shows up in my Inbox, I routinely logon to MyMarriottRewardsAccount to double-check my Awards activity.
What I really like is that Marriott has setup MyMarriottRewardsAccount so I can check from a Smartphone like my BlackBerry. It's especially convenient when I'm on the go. There's still one one more way to receive credit and that's to call Platinum Elite. You'll need your confirmation number, the date(s) stayed and the amount. Even w/e-Folio it's happened. (Sometimes the connection between the hotel and Marriott server goes down, I'm told.) Platinum Elite or Guest Services will call the hotel to confirm and assign credit.
I call the Platinum Premier desk and they take down some information over the phone. Assuming the hotel has the same record it is then added to my account that evening.
The Platinum Desk (same place with different folks answering) can probably also help and the call is free.