Individual spells are still cheap even if the blank slots must be purchased.
For example, 10 dev gets a list to level 10. In the old way, all ya got was a 50% to learn the list.
After level 20 there are more blanks, but the return is still excellent. Also, a mid to high level spell user can spend entire levels of dev learning spells since his lore skills and such are probably already well developed.
It also allows a list to be learned partially, say level 2 or level 11 for example.
I have also seen individual spell development in RMSS leads to knowing 15 spell list. Five list are developed 3 levels, then five other, then five other, then the original five are developed 3 lv3ls, etc. The result is 15 list to level, with some exceptions as list are learned piecemeal (Invisible Ways to lvl 4 is common, Anticipations to lvl 2 also, etc).
I started using the rule in RM2 with SUC. I was glad RMSS/FRP adopted it.