Except for popular vacation weeks and a few resorts that have some weeks being sold as fixed weeks (notably the new towers in Maui and I think one or two on HH; there may be a few others), most Marriotts are sold as floating weeks.
The advantage is that the owner gets to select from a block of weeks in their purchased season, and vary their vacation time from year to year as their needs fluctuate. The disadvantage can be, as pointed above, that some resorts have large season, and there may be only part of the block of weeks in the season that most people want (hence creating competition for those weeks). For example, NCV has a large Platinum season, yet most owners want those coveted late June-mid August school vacation weeks.
People who are happiest with the Marriott system are those who have some flexibility. If you buy expecting to always get a particular week every year, esp. if it happens to be a holiday week, you are apt to be disappointed some years. However, while you may have a first choice vacation week, if you will be ok getting a second or third choice, the system works very well for most people and gives you the option of changing weeks as job demands, children's schedules, family/friend events intervene.
While I know others love fixed weeks, personally I would not; it would be upsetting to have a wedding or other of a myriad of family events, to say nothing about job-related issues, which would force me to miss a week and have to deposit it for trading because I couldn't be away at a given time.
