If you're planning on purchasing points close to the amount needed for a fixed week, it's not really costing you anything to get the fixed week. For example, if you were thinking of getting 250 points, you could get a FW for a standard view 1 BR for weeks 1-3, 36-38, or 48. You're still paying the same amount per point for the 250 points. The additional "cost" is that if you actually use the FW, you'd potentially be using 10% more points to book than if you didn't (as pointed out above, it's possible the FW would actually be less depending on how the charts change). The FW gets booked outside of 11 months. You can cancel it every year. You can use it 1/2/50 times. You could get a FW for a difficult to get reservation and rent it out as a backup plan for years you don't use the points.
If the FW points are in line with what you're planning, the only downsides I see are 1) you would need to cancel the FW every year if you don't want to use it, which shouldn't really be an issue and 2) if you ever sold the contract, I think there's still confusion over how it actually works, which could turn someone off from buying. On the flipside, the FW option could result in a higher resale value, especially if it's a popular week.