I would prefer not to have 7/7 water in a shallow channel as well. Whether it took 1 or 2 is fairly arbitrary though. I would err on the side of 2 because of flow rate concerns.
Actually, I would hazard a guess as to say shallow channels would be easier to implement than pipes. At least with the channels such as the one I illustrated earlier they only imply a relatively minor water pathfinding addition for water below a certain depth. Aside from that they would act like normal tiles.
Pipes bring in other issues along the lines of their capacity, how they act when not full, do they teleport water straight through, how much water comes out of the taps etc. I expect they would be more likely to create inconsistent systems. Shallow channels on the other hand would likely be closely related to the system that we already have, volume and pressure are non-issues, the only main complexity that I can see is making sure it always paths correctly, which would be somewhat shared with pipes, minus the aforementioned problems.
While I did and still do view the 2D channels as in the Z-level below, the important part that I want to take from them is their pathing. The preferential flow system that existed in 2D made water path through channels first, then flood when filled. That same system could also be used for these shallow channels, with the addition of a check for depth first.
Also, as I said before, I'm not advocating shallow channels instead of pipes, both systems could be available. I would be interested to find out the history of piping though, to see whether it fits in with the setting of the game or if they became prevalent too recently.
jfs, that latest set of rules is the sort of thing I'm thinking of as well, although at one depth higher. There would also be preferences for flowing into normal channels, possibly down stairs and things. The preferences should be fairly logical to work out.