Would variable crank arm length be beneficial to a rider?
The system you propose is just a third way to change gear, in a mechanically very complicated way. Any benefit that could be obtained by lengthening the cranks can already be obtained by moving to an easier gear and a bike with two or even three front chain rings and anywhere between five and eleven rear sprockets has plenty enough gear ratios available to cover all situations. And if the range of gears available isn't wide enough, replacing the chainrings and/or rear cassette is a much simpler fix.