Everyone here needs to remember that Elite members, even Plats are ENTITLED to NO UPGRADES whatsoever! However, Gold and Plat members are entitled to Concierge or Club room ACCESS no matter what floor our room is located on, and we are eligible for an upgrade based on eligibility, suites are specifically NOT included according to the rules. All that being said, in practice, suites are available if you know how to go about asking for them! There are basically four steps to having the best chances to getting what you want. Step one is making sure that your Marriott personal profile is as complete as possible in all ways, including bed preference, floor preference, near or far from elevator/ice machine, etc. Step two is that I make sure to call the Plat rez line (instead of making most rez online) and at the end of making the rez, I ask the rez agent to put "request Platinum upgrade if available" in the comment section of the rez. Step three is about three days before arrival at the hotel, if an upgrade is important to me, I call the hotel and ask to speak with either "rooms control" or the front desk manager and inquire about getting the upgrade, and I specifically ask for a suite, if available. At that point I usually find out how heavy the occupany of the hotel will be during my stay, so then I will actually know whether an upgrade is realistic or whether I am being lied to. Then, when I check in at the hotel, I ask the person checking me in, what kind of room do I have. If they tell me that I have a suite, or that I am on the Concierge floor and the hotel is full, then obviously, I am quite happy about it. If not, I ask again for an upgrade. Sometimes the front desk person is not even aware that I am eligible for an upgrade (notice that I did not say entitled) as a Platinum member. If they are not aware of the Platinum benefits, I ask them to check with a manager whether there is an upgrade available, and why wasn't it taken care of before I arrived, etc. That usually works quite well, and the manager will even sometimes come out to apologize. I have gotten some amazing suites that way, including the Presidential Suite at the San Francisco Marriott, twice the second largest suites at the San Antonio Marriott River Center, a huge two room hospitality suite at the Marriott Wardman Park, and I could go on and on. The rules say no suites, but the reality is that hotels will give Platinum members suites if they are available, especially if those Platinum members are good, repeat, or long term customers of that hotel.
Bottom line, if you do not ask, you do not get.
Now, regarding KathyP's comment that "Marriott is notoriously tight fisted when it comes to giving upgrades." I do not find Marriott any more tightfisted about upgrades than any other elite program that I am a member of. I do not get upgraded every time that I am on USAirways flights, even when there are empty seats in First or Business Class. I do not get upgraded every time that I rent from Avis, even when there is a Premium or Crossover available as an upgrade to my regular Full Size 4 Door. So, I ask for the upgrades from both USAirways and from Avis, and sometimes I get those upgrades, and othertimes I do not. But, one thing is for absolute sure, if I do not open up my big mouth and speak up for myself asking for what I want, I do not expect that they will give me what I want every time voluntarily. I also examine my bill when I check out, because there are often mistakes, which to me is the same principle, you have to go after what you are looking for, the right amount on your bill, the right amount of points, whatever it might be, go get it, and don't expect someone else to get it for you if you don't get it for yourself. It is a learning process, and I learn things about the rules all the time, including what I can be compensated for if I don't get it, such as my welcome gift, or not getting my guaranteed room type, etc.