It's more time efficient, but is it more dangerous?
I think as long as any guards are in place and normal safety procedures are followed as mindfully as they should always be followed there's little increased danger. That's not to say there's none, and individual mileage as always will vary.
You list table saws, jointers, router tables and drill presses and I think this very much depends on each tool so it would be best to approach thinking about this on a tool-by-tool basis rather than looking for an overall idea of acceptable/preferable practice.
Is it better on the motor?
I think it can be, although it depends on the tool. And the key factor may be just how long it's left running. Industrial-level tools have very long 'duty cycles', where there may be an expectation that they have to have the motor running continually for an hour or longer at times. Consumer tools are not built as robustly and the potential for overheating when run for longer periods is higher, particularly in hot weather (temperature in the shop is definitely a factor).
Also the type of tool I'm sure must matter, table saws and especially jointers or thicknesses are obviously intended for working for longer periods compared to say a typical drilling operation.
But start-and-stop operation I presume is considered normal for drill presses so drilling, stopping and drilling again should be perfectly fine. And certainly you shouldn't be adjusting a workpiece underneath a spinning bit, that just strikes me as bad practice generally. Note: this is not to say that a drill is put at risk if run continually for long periods, many people use their drills for drum sanding, buffing, with wire wheels, even for turning operations, without apparent harm to the motors. But the quality of the tool probably matters a lot here.
With routers, they spin very fast and the motor and other internals are being stressed as a result so within reason I would want to stop the tool and let it cool periodically even if you might want to run piece after piece past the bit for extended periods, e.g. when doing long lengths of moulding. This may be unduly cautious but I would feel more comfortable doing it this way myself with a consumer-level router.