It really depends where you stay. Here is where I have recently (in the past year or so) gotten free breakfast as a platinum member:
Tysons Corner Marriott - platinum only
Fairfax Marriott at Fair Oaks - platinum only
Washington Dulles Airport Marriott - gold/platinum
Fullerton Marriott at CSU Fullerton - gold/platinum
Portland City Center Marriott - gold/platinum
Now, there are plenty of properties that do not:
Washington Dulles Marriott Suites - used to until about 12/08
Fairview Park Marriott - they stopped in 2008 (I think around April or so)
Redmond Town Center Marriott - they stopped sometime in 2009
Seattle Airport Marriott - they stopped sometime in 2008-2009
Gaithersburg Marriott - never offered it (at least when I stayed in 2006)
Cambridge Marriott - used to offer weekend coupons until 2008. Before then, they also had an omelette station in the lounge.
NYC Marriott Marquis - never offered free breakfast on weekends for elite members
I am not sure for a few of them:
Renaissance LAX - used to have a lounge open 7 days a week. When they started closing the lounge on weekends and cutting back on offerings, I got breakfast coupons a few times by asking I have stopped staying at this property, however, since I have tried to fly out of SNA instead.
Manhattan Beach Marriott - the thread on lounge access and breakfast coupons on weekends does not show them offering a free breakfast. I only stayed here once on a breakfast rate so I have no personal experience as for a regular weekend rate.
Philadelphia Marriott West - they used to offer coupons on weekends to elite members, but I have not stayed here in a few years so I am not sure about their current policy.
Others offer a discount to gold/platinum members:
Seattle Waterfront Marriott - full breakfast is $5 based on the thread for lounge acces and breakfast coupons on weekends. I have only stayed here when the lounge was open on weekends (in the summer when they have a lot of business).
La Jolla Marriott - (I should add this info to the thread for lounge access.) They offer all elite members a discount for dining (all meals) in their restaurant. It is 15 %.
The reality is that it will depend on what property you stay at.