It wouldn't break your character to level up through non-essential skills, but did slow down progress. That wasn't a huge problem because the leveled lists would still include the weaker opponents. Your character's performance and success were determined by the combination of skills and attributes associated with each task. Skills increased through use, and the associated Attributes also increased as a result, but you had a couple of voluntarily assignable points each level to tweak it a bit to your liking.
Oblivion was a MUCH bigger problem, where the low-level critters and equipment were REPLACED by higher level items as you leveled up, or SCALED so they were stronger at higher character levels. That weak goblin with a dozen hit-points which you killed with a stab of a rusty knife at Level 1 now has 600 hit points and takes 20 hits with a serious weapon to kill. If you leveled in a non-optimized way, the game got significantly harder, instead of easier, because the enemies got stronger while your own combat skills might not. Most Attributes in OB did nothing other than limit skill increases, so Attributes were removed in Skyrim as "useless". At that point, it was a cheesy console game system, despite the large open world.
Basically, M&B and Warband had a generic attribute leveling system, but you gained a good portion of your weapon skills through actually using them. Now it seems like they're trying to water the game down to Skyrim levels or lower, as nothing more than another generic action title pasted in a huge open world.
While I never like Elder scrolls, I did play FAllout 3 and New Vegas which are the same.
Comparing F3 with NV I enjoyed the latter much-much more. And I think enemies not levelling _at all_ is an important part of it.
At the beginning, there are areas which you should avoid, because the enemies (or radiation) there will kill you in a second.
At level one, a legion soldier is a terrifying opponent. (level 8-10 or so, with much better - but still only medium level - gear)
The brilliant part of New Vegas is that these are fixed: when you are level 30, the legionary soldiers are still level 8-10, so the tables have turned, you can wipe the floor with them. With a high level melee character (and good gear), you can go toe to toe with a deathclaw, and win.
I think this
1: adds to a more believable atmosphere in general
2: adds tension to the game
3: makes it possible for crafty players to take high risks for high rewards
I actually bought Fallout 3 GotY edition after New Vegas grew on me. It was a great disappointment.
You could beat a super mutant senseless on level one, with a wrench, losing about 10-20% of your health in the process. When you encountered a super mutant warrior 20 levels later, and attacked him with a super sledgehammer, you won again, losing 10-20% of your health.
Another game ruined by this kind of training is Obsidian's rpg, Tyranny. Luckily it has the lock down skill (= you forbid the player character to use it) option, which is not perfect, but if you understand the game, it helps a bit.