I'm in the same group as PeterR and Hurin with this one, as were all of the GM's that gamed with us back in the day and it ties in heavily with what Hurin stated in an additional post:
Games like Dungeons and Dragons need multiclassing because they have hard caps on what your characters can do. Fighters simply can't learn spells unless they multiclass (let's leave feats out of it for now, for simplicity's sake). But Rolemaster has only soft caps. Rolemaster fighters can learn spells-- they just have to pay a higher cost for them. So Rolemaster doesn't need multiclassing. And in fact multiclassing contradicts the RAW Rolemaster definition of class.
RM (in particular RM2 as I have the most experience with that version vs. the newer versions) allows anyone to do pretty much any thing they want either at a cost or with a penalty (-50 to the roll or something similar). Unlike AD&D where you simply cannot learn this skill unless you're a Barbarian, and you cannot do that skill unless you're a Rogue, and you can't cast magic at all unless you're a spellcaster and so on. The idea of multiclass works well within that system. That system was built up with that multiclass ability built in and has rules such as one cannot earn XP in the second class if the level difference is greater than 2 in the primary class or something like that. It got pretty confusing at times when changing classes but there are restrictions built in to help balance things.
RM is similar in that balancing in that your character can learn ANY skill he'd like if you are willing to pay the DP cost and to meet the requirements such as "non-organic armour interferes with casting" or "no head covering". We had one fighter learn a spell list once. It cost him 20DP for 5% chance to learn an open list to level 5. Eventually he rolled 96 on the spell gain roll and he could cast simple spells to level 5 from that list. Simply put, he's NOT a spell caster, he's a fighter, but he can certainly learn and practice spell casting in his free time and lo and behold, something clicked and he can cast a spell. RM has semi-spell user classes that offer a little of both worlds; slightly higher costs for the martial and magic, but considerably lower than a spell caster learning martial skills or a fighter learning magic skills.
I don't use professions at all, everyone is No Profession so this would never be an issue.
I've experimented with this as well with a small group of players. Every one was No Profession or was Professional, but the profession had to be one that was NOT already in RM2 books. A player couldn't say "My profession is Cavalier!" It had to be Baker, Blacksmith, Fisherman, or something similar. That was a really fun game. The townsfolk were left defenseless when the king's men and sentries were recalled to the capital to defend against an attack and the outlying towns were left to fend for themselves. It made for an extremely fun campaign and really interesting character skill choices.
I would note that warrior monk -> magician is probably the most abusive profession change available with the core professions, any way. As a warrior monk you can quickly build Adrenal Defense and Body Development (and some other skills apply to this, but those are the main ones) that patch the weakness in the "glass cannon" design of the Magician.
This is why we (my group of players/GMs) didn't allow class changes. How unfair would it be to be a Warrior Monk buying all the great fighting skills at the lowest DP cost in the game, then switching to a spell caster to buy all the spells costs at the lowest DP in the game, then switching to Assassin to buy all the stealth skills at the best price in the game? Why not just say every skill in the game costs 1/2 since you're just going to switch to a new Profession when you want to buy a skill that is too expensive? Adrenal Defense too high? Be a Warrior Monk for 1 level and buy up the skills. Sense Ambush Assassin cost too high? Be an Assassin for 2 levels and buy up those skills.
The balance is there. Some folks have an affinity for magic and it comes to them more easily. Training in martial skills means learning weapons skills are picked up more easily. That doesn't mean the fighter can't learn First Aid and Second Aid and Surgery. You just may not want HIM to work on you vs. a trained Healer.... but if you're in a battle field bleeding out, I'll take that fighter with the mediocre skills vs. the Maleficant with no healing skills at all!
This is our group's take on multiclassing. Others have posted here and have shown that it works for them and their group, but I think this says it best.....
That is OK, what he thinks is largely irrelevant as he is not running the game.
Of most importance is do you as GM want people to do this? If the answer is no then tell him that he can learn open lists to 5th level if he pays the points to learn them. Take it or leave it.
If you are happy for him to do this and you accept than none of your players will probably ever play a straight pure spell caster again then you just need to work it into the story....
You're the GM. Does Multiclass exist in your world? If you allow it, all of the other players will start fishing for the professions with great DP costs to bolster their weaker/DP-expensive skills. Everyone will tend towards Fighter/Spellcasters to get the best of both worlds. Has this player ever GM'd a session of RM? I found that once I was on the other side of the screen, a lot of the "heavy handed decisions" the GM handed down made a ton of sense once I saw the internal workings of a gaming session.
