I remember the ladders being really buggy in the closed alpha. Bots also had no clue how to use ladders which made for some balance issues. I guess they were cool, but maybe they were just too much a of a hassle. On most other maps the devs put stairs or other means of vaulting onto spots that previously had ladders which satisfies the whole More verticality thing, though I must agree that on oilfield specifically, not having useable ladders is pretty odd considering the whole place should really have ladders.
I hate ladders. Ramps and such make more sense. Ladders are a ridiculous mechanic that just make you unable to shoot the entire time. If you were ever in a tactical situation, noone would climb a ladder unless it was the absolute only option. Ramps and such allow you to be strategic and shoot and still get where you are going. Removal of them makes sense on many levels (including how buggy they are).