Don’t forget the impact a driven front wheel would have on steering and control.
There would be an increase in gyroscopic forces that would make the steering heavy and awkward.
I’m not sure it would, those hub motors used on things like the Ribble bikes are tiny. Barely bigger than a dynamo and put out 250W. It might be noticeable as unpsrung weight, but not enough to be detrimental I’d have thought. No one ever decided SID’s were better than Lyrics because the lowers weigh half a lb less.
A better control system might be a sort of reverse-abs that adds extra power up front (in addition to the normal torque/cadence sensing on the cranks) when it senses the rear wheel moving significantly faster.
To have 2wd, you’d either need two batteries, meaning even heavier, or run off one battery, and deplete the range.
Same power output, just split between two motors. You could probably save some of the weight from the ‘main’ motor too if it was an integrated system.
In the scenario you describe, which I may have experienced once or twice in 30+ years of riding, I guess it might help.
You’ve only spun the rear wheel out on a climb, twice, in 30 years?
Roots, rocks, steps, muddy bits, none* of those have ever brought you to a standstill because the back wheel spun on them?
Having a bit of ‘pull’ to offset those times when you have to back off and unweight the rear of the bike over stuff would be pretty cool.
*well I suppose one or two of them might have contributed to you’re only 2 failed climbs.