I have an old car (but with electronic injection) that predates by far start/stop systems which are used nowadays to save fuel (turning off the engine brings savings after 3-5 seconds, see Calculating engine starter’s energy use ).
Does it makes sense to start/stop my car manually when I expect more than 5-10 seconds idling? I noticed that starting the car when warm is almost instantaneous, like 2 seconds.
The starter motor may get additional wear, but after all I rarely hear of failing starter motors... it's a tough component anyway, and there is always enough time between idling to cool down the starter motor.
For info: the starter motor of my car failed at the 181000 km mark. I still have to check, but very likely due to worn out brushes. This with normal use, no start/stop except sometimes in the last 20k km, so almost nothing. Increasing the start/stop cycles by let's say 5 (meaning 4 start/stops when idling per driving session) would have reduced its life to 30k km, not counting the remaining issues as pointed out in the comments. So, not worth at all.