Possibly, 5 or 6 units in a tile act as a ramp so one could stack crates to climb a wall, for instance.
I like this a lot -- particularly for the image of goblins climbing over the corpses of their fallen comrades and pouring over your walls.
One (potential) issue, though. If you have 5 obstruction in one tile, and 5 obstruction in an adjacent tile on z-level up, can you move between them? If you go with the metaphor of stacking objects up to climb on them, this is like moving between two unobstructed tiles on different z-levels, so making 5 obstruction == ramp doesn't quite work.
If you model the obstruction like fluids, with the number corresponding to height (and a tile having a total height of 7), then you could have a traversable height distance (let's say it's five). Then you could climb up a 5-tile, but not to the next one since there's a total height difference of 7 (2 in your tile + 5 in the next tile). So you'd have to have the tile in the next z-level up be 3 or less obstructed.
One other thing with this "fill height" model is that an underground tile that's full to 5 only has 2 height remaining, so a dwarf would have to crawl through, increasing the movement penalty. Outside, though, the penalty would only be incurred for climbing up the stack since you can walk freely once you're up there.
I'd also like to note that I agree with Draco's [SIZE] suggestion, but think it should be quantified in terms of the familiar 0-7 like water and magma. Much like clutter in workshops, it would take a certain number of objects to increase the obstruction level by 1, based on size.