I, personally, don't really like it all that much, but it simulates channels more realistically, have any of you ever tried to mine a straight hole with a pickaxe? Nether have I, but a pickaxe is round, so unless your dwarf can bend down and yet not fall in, it makes more sense to have ramps.
As a matter of fact,
I have.
The old way was far more intuitive to anyone who has ever dug a square channel.
You swing parallel to the wall when digging the bottom edges. I prefer an adze, pulaski or a hoe-dad and a post hole digger(it's much easier to cut the earth away and lift it out in a single stroke with a wider blade), if you want really square corners in your channel, but the job can be done with a pick, or if you are really creative, and the channel is wide enough a shovel.
The sides will be a little rough, if you don't get in there and smooth them, but we already have something for that, don't we?
Granted, it's much faster to dig a channel if you can stand in it, but that means channeling should take a little longer than tunneling, or maybe even slower than digging a ditch and then cutting the ramps out of it, not be completely replaced with this ditch system.
Realism aside, the biggest problem we have here is that the dwarven AI is not up to dealing with ditches that contain hazardous fluids. If the ditches themselves aren't a bug, the way dwarves run through them is.
The ditch thing has got to be a bug--Unless this is all Toady's April Fools joke, and we're all fools.