I agree.. seems like Apple derped there. The auto-correct was because typing on a smartphone is a lot harder, but some genius must've thought "Hey, why don't we use this software on our desktops too?". Also holding down a hardware key to get to accented characters, oh fuck, how is that ergonomic?
I think it's a good addition for when you very occasionally need to type a particular accented character, since you can visually see the options. Of course if you are going to type it a lot you're better off learning the Option key combo.
For instance I would have to look up how to directly type "ł" since I rarely need to type a Polish name... but I can get it in 1 second by holding down "l". Very handy!
What's more, you can't even type ł on the standard Apple layout. Sure, you could use the Apple Extended keyboard layout that handles more characters than what's used in Western Europe, but you'll lose ≈ (irritatingly, the Extended layout isn't a strict superset of the standard layout). I type ≈ more often than I type ł or ć or ō, so the hold-key behavior is a handy addition.
For anyone else having this problem, the new MBPs ship with the same autocorrect "feature" that comes on iOS where when you press spacebar twice (or, I should say, your keyboard thinks you pressed the spacebar twice) it inserts a period automatically. In addition, newer MBPs automatically capitalize and autocorrect spelling, like on mobile.
You can disable this. I did, and, I don't think any desktop computer should be telling me how to type. I don't need help when I have a full keyboard. System Prefs > Keyboard > Text.