Having HLA spells as abilities instead of top level spells

I think there is an SCS option for this right?

What are the thoughts on this generally?
It seems it is a trade off since with it:
- Sorcerer cannot summon Planetar after Planetar (activateable only once)
BUT
- But your F/M that made it lvl 18 finally does not need to trade its singular cast of Time Stop to choose between a Planetar or Comet or whatever.

Is the SCS compromise balanced or should it be viewed as unfair?
(How is it in PnP?)