The thinking, by those who think that way, is threefold:
(1) If you spend less than $15 mil on Adv, your numbers start to be pretty far off anyway.
(2) You get $185 mil, and you have to save somewhere. This is one thing you can get around.
(3) Once a player has a good 1-2 seasons in the minors, an experienced owner will have a very good idea (in general- with a few exceptions) of where that player will max out at based on their progression. In fact, probably better than the projections, because you won't get fooled by guys who are going to fall way short of their projections.
(3a) The exceptions: DITRs are hard to account for using the progression method, and major injuries or one season of bad misuse by the owner can also throw things off a bit.
Major downside: Much harder to make trades involving kids in their 1st season or two in the minors, because you can't see how fast they are going to progress.