IIRC Toady made this change not as an attempt to "improve" channelling, but to make channelling much harder to use as a quickly creatable superdefense.
Given that you have over a year before the first attack you have plenty of time to dig out some channels, smooth out the edges, or even, as I did once in 40d, build a moderately large multi-story wooden fort with external palisade, outbuildings, and a drawbridge over a pond on the surface (I accidentally forgot to bring a pick
)...
The almost imperceptible change in time it takes to create a "superdefense" considering the enormous amount of time in which you have to build it renders such an excuse meaningless. On top of this is the very real problem of lost miners and equipment due to liquid breaching and lost functionality and you have a change that has had a net detrimental effect.
Personally, I decided to learn how to be careful in order to avoid so called "ninja trees" (I've never had one fall on me), trapped miners, and other things. I also took the time to understand the z axis so designating ramps from a floor down does not bother me, especially since you can start the designation from the floor above to get the shapes you want, or at the very least create the basic guide around which you build your non rectangular shaped hole (My dwarves like circular rooms these days).
As for aesthetics. Once you master not having "fun", you turn to building vast megaprojects that are entirely aesthetic in value. I have a few plans down that route myself for the new version (Yay, sold a cow for magic beans!).
It just seems absurd to expect that a game this ludicrously complicated and as delightfully murderous as dwarf fortress should coddle you when you could just be careful.
For now, I mourn for the lost functionality and lament the current silly redundant channel/ramp designations that do the same thing.