Hiya!
Phew! Lots to get to before I head of to bed...
A Rogue learning "Cunning Action" is a Rogue thing. A Rogue NOT learning "Cunning Action", but in stead knowing how magic works and being able to cast Cantrips and 1st level Wizard spells...when 8 hours ago he was finishing his dinner and beer down in the common room. HUGE difference, imnsho.
I beg to differ. Having something like this would do the trick, I think...
"In Game Considerations
Multiclassing can not just be taken on a whim. If you are planing on having your PC learn a level in another class, you must inform the DM at least one full level before you take on a new Class (e.g., a 4th level character could not add a new class at 5th level, but could at 6th). This gives both you and the DM time to work in just how the character learns the arts of Wizardry, or suddenly finds himself having visions from a particular deity, etc. This only applies to adding a new class, not another level to a class you already have. Remember, the Multiclassing rules are here for those who want to create a character who's abilities and skills don't quite fit into the Class system as presented here. With Muliclassing you and your DM can create unusual secret orders who's members are trained in the arts of Fighting as well as have innate magical abilities as Sorcerers, or perhaps there is a clan of Barbarians who have special individuals who are trained to use gurrila tactics of hit and run, communing with animals, watching the stars for portents, and speak in the sacred language of the Druids."
There. Now there's a rule. It fixed a "RP'ing problem".
It's not a matter of "it doesn't usually happen like that"...it's that there is nothing in the rules that prevent or even really deter it. As I listed in my example above, they designers could have come up with something to make MC'ing this way a little more...sensical. At least to my old eyes anyway.
Just like Feats, I'm glad others use them and have fun. I'm also glad that MC and Feats are both listed as OPTIONAL. But for me (and my group), they just don't. Our games run fine without Feats and Multiclassing, so...yeah.
^_^
Paul L. Ming