Honestly, I can't believe the first option got more than one vote. How can you be smart enough to figure out how to play DF and too stupid to realize you could carve up ramps to achieve exactly the same thing as the new channeling?
Because designating your down ramps from the same level, rather than having to shift down a z level and designate up ramps, is easier on the players, and frankly, more intuitive.
The "dwarves in the drink won't auto-drown" isn't really a bonus, since you could always have cut that ramp or stairs in the first place. In my experience, having a stairs or ramp for access will only save a dwarf in a moat or pond IF the ramp/stairs is close enough for him to reach and get out. This means most of your dwarves knocked off in the drink are going to still die if you cut out all those ramps out and only left 1 access tile so you could get all those stones out of your moat. Of course, if your dwarves can get out safely, so can whatever else is knocked into the drink--- like ambushers and seigers.
Also, it is more logical to some people that the digger MUST be in the same square as what he is working on when digging DOWN. They think the digger stands in the square, and attacks the floor with his pick, ending up on the level below when channelling. Personally, I don't believe that is true, due to the fact that for the other forms of digging, the miner stands outside the target square, and ends with the miner still being in the square he started when done and he's mined the whole tile out. If a miner doesn't have to enter the "dig target" square he's digging sidewise to reach the far side of the tile or its far corners, then he shouldn't be required to enter a target square when he conducts the same dig command, just because it is downwards. It seems a tile should be approximately the same height of area as it is width and length (a (near) cube, rather than a taller then wide/length rectangle).
But even if the tiles are "taller than wide/lenght (ie, a tall rectangle)", when a dwarf carves a stairs down, or an up/down stairs FROM ABOVE, they don't enter the dug square itself to finish the bottom of the stairs or finish mining out the far corners of the tile. So again, if you can dig a Down or Up/Down stairs from above without ending in the target tile, I don't see why digging a channel should require you end up at the bottom of what you dug. That's a basic break in DF consistancy for a the task at that point. Or, as some have said--- it's less realistic, because you MUST be in the same square you are working, so all the other dig methods should work the same way. DF should be internally consistant at least.