That is sort of my problem, I don't think they are weak, I just think they are incorrectly percieved as weak.
Indeed.
I think part of this is the perception (and the player's deception) the other part of the threshold in many skills (and spells, most open and closed spell list are designed to have a threshold at lvl 5/6). On the other hand RMfrp doesn't claim to have perfectly balanced professions. It's the job of the GM and the setting.
A lot of the pure spell user skills (spellmastery for example) become super powerful, after a hard time in lower levels.
The probably most powerful profession is the paladin, but he'll be balanced by his deities rules.
Magicians are often part of a school, union, guild, order, bondsmagi, etc.
And higher spells also increase the risk factor (very nice way to keep a pure spell user from spamming spells).
To manage the low pp issue in lower levels, they often get rings as novice, adept, master ... our GMs used to give pp repository rings to a novice or a staff with charges. (or a better quality weapon to the pure arms - weapons tend to break anyways). So at the end many characters are not really more powerful in lvl 7 than in lvl 3 in their main proficiencies.
A 'of Mentalism' pure spell user isn't the best example for the melee possibilities of pure spell users, they have significant lower costs in mundane skills as well as the possibility of metal armor.
My longtime experience is that the pure arms or arms based semi have a more unreserved life and a bigger bunch of contacts and buddies than the spell users. Who likes to go bar-hopping and end up as a frog next morning because his buddy became too drunk?