Hiya.
*shrug*
I think the 'borkeness' of things is starting to show up more and more because the designers of 5e have slowly (over the past 8 months to a year or so) drifted farther and farther from "Base game to add your own stuff to", and closer and closer to "Uber-kewl powerz and lazer-cats!...pew!...mew!...pew!".
What I mean by that, is simple; it's easier to give, than receive. If Multiclassing had one of the first sentences say something like "Multiclassing is an option that some DM's may want to make available in their game. Here are a list of typical multiclass combinations that work well together. [insert list of class-combo's]. Of course, a DM may wish to open all classes to all combinations, but this option can produce some odd, unusually powerful or unusually weak characters". Problem solved.
Its FAR easier for a DM to tell his group "Ok, I'm letting Barbarians multiclass now. You can substitute Fighter with Barbarian for any of the listed combos"...or even "Any class combo is fine for this game. It's wide open". But going the other way? Where players are *expecting* to be able to make whatever monstrosity they can dream up, and then be told "Uh, no", by the DM? Lets just say in the first scenario, everyone has smiles on their faces....whilst in the second one, everyone has frowns.
(would you rather win $1,000,000 and be told that after taxes and whatnot you get $100,000...or would your rather win $100,000 and be told it's all yours, free and clear? Same result, but different attitude).
^_^
Paul L. Ming