Nonetheless, that's one of the things that irks me so much about RMFRP vs. RMSS. You buy "Spell Law of..." and pay again for the last quarter of the book you got with the last one.
However, I agree with you about the spell lists. That's one of the major selling points for me, too. Scaling spells would be nice. I can understand, however, how they might be somewhat mutually exclusive ideas.
One of my friends is mostly sold on the idea of playing RM, but he keeps asking if there are other options for the use of magic. He dislikes a certain other games' "Arcane/Divine" split, and RM is only worse in that regard. I sat down last week to port Ars Magica's system over, feeling that it would be the easiest to merge into RM's skill system, but I didn't have much heart for it.
Using RM as a toolkit, though, could be beneficial. Give our GM's and gaming groups several choices across the board, let them choose what they want to use. Two or three stat rolling options (I use the RM2 method myself), two or three spell casting styles, and so on. I'd probably have an even easier time selling new players on the system if they knew I could build a system they'd like, or even replicate a game they like a lot, with RM style.
That's one of the biggest things I like about RM, it is (usually) easy to swap different systems in and out. Maybe a potential revision should try to embrace that?