In depth review of Con #2 of Buying a Timeshare FLEXIBILITY

Flexibility

Although there are timeshare plans that afford more flexibility than others, some people feel nothing is quite as flexible as being able to decide when and where you go each year, in the more traditional methods of travel planning. If you want a holiday week, you may not be able to trade your timeshare for it, because there are many timeshare owners hoping for the same week. You also may not be able to decide last minute that you want to use your timeshare in a certain location.
If you buy a floating week timeshare, you will have more flexibility in when you use the timeshare, but availability is given on a first-come, first-served basis. So, planning farther in advance serves you better.