Glad to see there's at least some support. I thought the uses would be immediately obvious, but here's a few I'd like to be able to do:
1. as others have said, have magma/waterworks with less loss. right now if you want to move water around using slopes, you end up with lots of evaporation. That isn't to say there aren't workarounds, but workarounds aught not be the mainstay method forever. Also, liquids have no preference to actually move down a slope. If I have a long slope running down the side of a mountain, water dropped at the top isn't going to travel down to the bottom, unless the slope is turned into a canal and one were to simply dump so much water that it was forced to 'fall' off the sides until it got to the bottom.
2. moving stuff around. who wouldn't want to build an awesome log flume to get lumber from the wooded hills down to your sandy plain. Or for that matter, build a dwarven waterslide. I know mine carts and other forms of material & dwarf transit are very desired features, and this would be just another (and IMO even more dwarvish) way of doing it.
3. deathtraps! like an earlier poster said, pulling back that bridge would be that much more satisfying if you could have the gobs/items fall down a funnel o' doom.
Ramps -do- already take into account direction, and there are a number of different ways this could be implemented.
And even if it does prove to be 'more trouble than it's worth' (a call I think only toady can really make, since we don't know the specifics of how the system handles these things, just generalities), the game has always leaned to the 'as realistic as possible while still fun' side. And I think the myriad possibilities having real slopes would open up would be worth a shot