I guess I could make a guess, but it would be just a guess (Didn't know I could use the word guess three times in a 14 word sentence).
Let me rephrase your question: What is the downside of buying a Fixed Week deed versus not buying a Fixed Week deed?
Let's say I want to stay in a Lake View Studio at PVB in early December most years. A Fixed Week deed guarantees me that I can ALWAYS get a studio. So IF PVB becomes a hard-to-get accommodation during my travel time, I've eliminated any risk by locking in the Fixed Week.
But let's say PVB isn't hard to book at 11 months. What is the downside with buying a Fixed Week? Well, not much. A Fixed Week deed for Week #49 will cost me 163 points, while a traditional points deed will cost me 148 points. 15 extra points is not cheap ($160 * 15 = $2,400), so that is part of the downside of a Fixed Week. But, remember, I still own 163 points. If my week isn't in high demand, then I can always opt out of the Fixed Week each year and have 163 PVB points to use. I could book the same week for 148 points and then use the extra 15 points somewhere in the
DVC system. (Note: Anyone thinking of employing this strategy of opting out of a Fixed Week should first make sure they can use the extra points. 15 points won't get you anything at PVB; the points would have to be used for a single night at another resort, or combined with other points to book at the 7-month mark).
If I was thinking about buying a week's worth of points at PVB, I'd give serious consideration to buying a Fixed Week. I'd probably opt out most years, so I would be utilizing all the points I bought. If it turns out my week is hard to book at 11 months, then I am protected.