I like the fact that, in Rolemaster, your base spells continue to be relevant and important no matter high level you get. Basing the RR on the caster level and not the spell level is important for that. Although you have access high level spells, because power point costs increase faster than your available PP, you're never going to be able to rely just on those high level spells, they will always be something you have to use carefully. That's not true for your target's improved RRs due to their higher level, which applies on every RR.
I like that your higher level spells are related to your lower level spells. It gives some coherence to your character, rather than just being a bundle of random tricks. Spell lists achieve that, slot systems don't.
I like that you pick up interesting tangential tricks along the way, things that are related to your important spells but maybe things you might not have actually selected. You may not use them very often but they can spur creative solutions. Spell lists achieve that, other systems for making spells related (e.g. spell prerequisite trees like GURPS) don't.
Long spell durations and such for high level Arms characters are a bit quirky, I don't know about that. On the other hand, it helps keep those spells relevant at high levels. In some other games, if you pick up a 1st level spell somehow (multiclass, feat, racial trait, etc), it quickly becomes irrelevant and pointless. If it was a choice between giving the Arms character duration/range/area based on their level vs based on their number of ranks in the list, I would stick with the current system.