So are steps but we use highway=steps. Steps and ladders are, of course, a barrier to some. But, actually, their primary function is to make traversing something accessible (to many). Highway make sense for both steps and ladders.
Edit: sorry, I realise I’ve just echoed what @crodthauser said.
Nah, it was @Adam_Franco who brought that up, I replied there just to let anybody know how much a nuisance I can be. BTW. how about highway=stile? I helps you cross the fence/wall/whatever barrier there is, not?
When browsing photos, I happened over this one:
I just tagged a highway=ladder here - Way: Hechenberglsteig (1257000571) | OpenStreetMap - curious, how long this will last. Also downgraded sac_scale from T3 to T2 there. T1 would be too much of an understatement, in my opinion. If there was no ladder there, this would be a highway=scramble, in my opinion 
