Oh one of my Gm's refused the Healer class to be in the game (for a while) because of caster/armor abuse. I created a healer and was laughed at for getting a talent to help me develop my weapon skill and also to help me develop my armor (for a Healer). Of course I developed heavy armor and a long sword/shield. And everyone laughed at my guy (as it took me many levels before I could even wear heavy armor). I was also laughed at for developing the closed Cleric list Life Mastery (so I could rez dead people). This tied up alot of development points.
However, around lvl 12, my healer was a monster. Being a Channeler he was able to use transend armor and he was quite able to defend himself and heal everyone around himself. While the rest of my group was dead/out, the "weak mocked" Healer defeated the enemy, and then healed up the group...of course being a healer he had to rest in a tent for a week after as he laid there healing himself (I had to take on alot of their wounds to quickly and efficently heal them, so they wouldn't die of their wounds after I rezed them).
Being Healers are Channeling/Mentalism they seem built to skirt the armor rules. Transcend armor and being able to cast in metal armor as long as you don't wear a helmet helps alot. Getting a mentalist "helm" or head band saves from the crits.
But if the rules were ignored it could be pretty ugly. A warrior could have a real tough time taking out a Essence user wearing full plate and ignoring the armor penalties put on his casting (but if casters can ignore armor penalties for casting, can Warrior types Ignore casting limits/rules?). Or imagine an Arcanist ignoring those rules. But remember the ruleset is set so that an essence user could wear heavy armor if he was willing to develop the skills and cast with the penalties. This is Rolemaster you can develop all skills (at a cost), no matter what class you are.
I personally feel the ruleset is very heavy handed on Channelers though. Firstly their professional lists may not have any heals in them (this means a cleric has to develop 12 list to remain basic as Rolemaster requires alot of list to truly heal). On top of that Armor Development cost are very heavy. And then they have to develop transend armor which is a restricted skill (which seems counter intuitive since your god gives you the spells). Healers don't get many instaneous cast heals spells (which can be really ugly for the group).
On top of that... The Channeling Companion has all kinds of rules to limit (take healers power points away on the GM's you've been a bad healer whim). Oddly enough these rules don't seem to affect Animist, Rangers as much (as they are only worried about the forest in most GM's mind). Then if you add in Warlocks and the Faith Breaker list and it becomes a how many days will my healer be without power points, or i'm instantly corrupted and now hated and hunted down by my own faith kind of situation. I don't see other caster types having to deal with this kind of high development cost/ instant loss of power situation.
Other casters may not have armor, but they are not as vulnerable to instantly losing their power points or their spells. The anticipations list can be just as powerful as having armor. When used with armor it can be crazy. Ignoring the Armor penalties on top of this seems it would really put the rest of the game (especially healing) out of whack.
But as many have stated Mentalist can already cast with armor (without much penalty if any) as long as they are not wearing a helmet. Channelers can also wear Armor and cast through the Transend armor skill (I.M.O. needs to be fixed, it's way to costly). And if an Essence/Arcanist type is willing to do it, they can just spend the development points, stay the course and cast with said penalty. It is part of the rules. So essentially all players(including warriors with a mind to do it) can deveolp casting ability and armor wearing ability.
Bottom line to me though is the restrictions of Essence/Arcanist being affected by metal armor seems logical as they have the most powerful "attack" list. Channelers have to maintain a good relationship with their god to be able to "channel" through armor or get any spells. And Mentalist have to cast without a helmet (I did it with a warrior of mentalism realm, died from a crit after doing the same to a tougher foe) so it is not without it's dangers.
So in my mind Armor Penalties for casters need to be there. To me it is a balancing issue, and undoing it could really shift the balance of power heavily in the favor of the casters types. Especially Essence users.