I think alot are missing the point PROMETHEUS_ts is getting at.
Whats the point (aside from RPing) of playing a pure mage when a warrior can be just as powerful at magics, only also be strong at fighting. The more pure a class you have the more benifits you should be able ot get....or the reverse, the more multi-class your character is the more disadvantages you have to face. if you look to other older classic RPGing (Neverwinter Knights for example) the more you multi-class the more disadvantages you face, while also getting extra abilities to make for it. This allows some multi classing....but it stops you being an alrounder at everything.
Just answered your own question right there. As for multiclassing penalties, that's a purely D&D trope, and likely because all players could only invest the same amount of time into their character progression. In real life, you can have mastery in as many disciplines as you have time, energy, motivation, and talent for. This is why, every single day, you see some people that can master a wide range of things, and others who can't.
A real life example....should a master at martial arts be able to be outclassed in a fight by another master of martial arts....that also is a top scientist? The guy who has devoted his life to training a single skill should be better at it then another who has spilt his time at learning two.
Why not? Again, it happens ALL THE TIME. Not everyone is equal and this goes for both latent talent and motivation to excel. Some people are better at learning things, some people are more naturally predisposed to a various skill. Some people are both. Some bypass both by sheer hard work. But if you look around you, its easy enough to find a plethora of examples of people who excel and master multiple disciplines.
If they take the current ES route, then picking a class will only matter for the first 30 levels....after that point in any ES game you start being a jack of all trades....able to use a sword to kill a masterful oppenent, pick impossibly hard locks and cast powerful spells.....leaving no real point in developing a character.
ONLY if you take the time to practice those skills. If you aren't paying the price in practice, those skills are not advancing.