Mass-promote at the end of the season after each respective minor league level is out of the playoffs, and before the ML world series ends.

The next time one of your worlds is wrapping up, take note of the development cycles. Every player will have either 4 cycles labelled "regular season" or 5 labelled "regular season". If you promote at the end, all of the guys with 4 cycles will get a 5th cycle labelled "promotion" and that 5th cycle is usually associated with a boost.

It is not clearly understood whether you would lose that 5th boost if you do not promote, or whether the "remainder" carries over to next season's spring training. I am inclined to believe that the remainder gets rounded off and you lose it. Why bother guessing when you can just guarantee you get it? Guaranteeing this +1 for each of the player's first 3-4 season (including the introductory pro year: 0) makes a difference of about 3 OVR by the time the player reaches the majors. +3 PER PROSPECT... this makes the difference between AAAA and ML bench (cheap labor platoon guys), between zero compensation and type B, and between type B and type A. Over the long run you'll surely notice it.

I have also observed in certain instances that I have gotten a 6th boost. One season I had a low-A team miss playoffs, so I mass-promoted the best low-A guys into High-A and they got the 5th boost. high-A made the WS so the player(s) played in 20 extra games or whatever and at the end of high-A I promoted into AA and the player got a 6th boost which was only about half of the 5th boost (because of games played).

Basically the designation of "promotion" is that you are getting a regularly-scheduled boost a few cycles earlier than you should

It's not a guarantee that all 4s will get a 5th but I agree with your sentiment on why not do it.