Also, you're wrong about the irrelevance of low odds. Low odds means that it will happen infrequently and thus dwarven deaths will remain below an acceptable threshold.
That's just another way to say "It's OK to be less than perfect".
NEVAR!
You aren't truly playing Dwarf Fortress until you have pathologically installed a countermeasure to
every conceivable occurance as well as forged it all within a perfectly symmetric geometric fractal fortress. Floors will be paved or entire layers obsidian cast for symmetry! Only then will you "finish" a fortress.
EDIT: More seriously, though, I prefer running my forts in the background while I do other things. I value automation above all else. I want to fuss for days over getting every minor detail right to set up a system that works without me having to constantly correct it, and then never have to touch that system again because it
works.
If you are surprised by anything in DF, you have not done enough Science yet. Everything is foreseeable, and every problem can be forestalled.
Casualties of any form but old age are a sign of failure to predict and plan.
Regarding combined functionality constructions and stacking, I don't think that's entirely necessary for any partivular purpose, since you can use directional speed addition from the rollers to direct carts into and out of holding loops. If a stop could provide directional momentum on mechanical trigger, that would make things easier. But stacking would mostly just reduce space needed. Plus it doesn't make much sense from an in-universe logical perspective.
It's as logical as a stop and accelerator could be in the first place to combine the two. It simply means the stop retracts, and the accelerator starts up. Alternately, you could use the same gears/wheels used in the accelerator as the stop, as well, by just "shifting gears" from one in a high-friction stopping gear to switching over to a powered gear that pushes a cart along.
EDIT: Alternately, an unpowered accelerator could simply provide a drag that makes it functionally the same thing as a stop, and that solves both problems. No power - stops cart. Power - pushes cart along. You just need to create a cart course that requires no manual stops or starts.
Powered accelerator stops are also nearly mandatory for full automated cart tracks, as they allow you to push a cart without having a dwarf present at all. Otherwise, you have to have a dwarf actually sitting there pushing them. Who wants that? It's like the difference between setting up your traps on a repeater or asking a dwarf to try to sit in a control room and pull all your levers constantly - they take breaks and aren't always responsive. Full automation means full reliability, and enables perfection.