I suspect the RM2 PP structure wasn't completely askew, it depended on the very game balance issues in question here.
a spell that nerfs locks or traps could be viewed as having the mage infringing on the thief character's concept.
But, if you need to spend a chunk of DP to nerf said lock or trap, you might want to instead save your PP and allow the thief to pop it.
Thus, in core RM2 or RMC the mage is often too cheap to be "doing everything all the time" as they were PP poor.
Now, you high magic the game, by tossing a x3 PP multiplier in there, and suddenly the mage is doing everything.
shrug, you play in a high end, high magic game, I fully expect that in fair value to the x3 multiplier the thief has a cloak of invisibility, a magic climbing rope and some high bonus armor/weapons. . . .is the thief now stepping on the mage's toes as he scales right up sheer surfaces while invisible? I don't think so, I think it's just in a high end game all the edges get blurry.
if, OTOH you hand the mage a x3 multiplier, and the best the thief can seem to get a hold of is a set of +5 lockpicks of quality and a +5 dagger, then the problem is that as GM you're way favoring the mage over the thief. . .the mage isn't too powerful, the GM is biasing the game in the mage's favor.
Back to the "no multiplier" core RM2/RMC game. . . .in my experience, the lack of PP made the mages tend toward lores, to give them something to do 80% of a session. . .not casting or combat, just "These glyphs are ancient denari, and mark the tomb of a tribal chief." or "This is an unusual metal, I can't identify it but it's neither steel nor silver, despite it's color.". . . saving the firebolts or lore spells for truely critical moments. . . . .Nobody seemed to object in those "dung and beans" games. . .since when pressed, said mages could still fly, or blast an area, or delve the story of a dead body. . . .they just didn't do it so often, focusing more on being the well educated lore wise party member and mostly avoiding combat.