In a way I agree, just picking a random week isn't likely to help in any meaningful way. But picking a lower
point chart room in early December for example is a pretty safe bet.
But, if there is a decent fixed week for around the number of points you want to buy, it doesn't cost you anything up front to add a fixed week. You only "lose" some value when you actually use the fixed week (and depending on the week, it may actually save you points if it was a cheap week when the FW chart was made or if Disney has raised the point cost of that period after you bought the FW).
It DOES cost you more to split a contract up into smaller chunks. So in a way that is actually more of a gamble. If you never sell, then that money was technically wasted, but if you got a fixed week but never used it, you never wasted anything.
It's just another option you can at the end of the day. How much benefit you get out of a fixed week vs splitting contracts just depends on what fixed week you have and if you ever end up selling it. There is no way to know how Disney will adjust the
point charts ahead of time or if you will end up selling the contract in the future. It could be considered a "gamble" either way.
I kind of did a bit of both with my 300 pt direct purchase to get to the best incentive level when I bought. 2 fixed weeks in a harder to book standard view at RIV + 1 small contract. 1 for week 39 during a "discounted" week that actually costs less than booking normally during Food and Wine for Epcot access, and 1 week 50 during early December when booking is tough. 114pts, 135 points, and a 51 point extra contract. I don't imagine any of them would be that hard to sell but now I have various options to use, keep, sell, etc. in the future.