If Flettner rotors (spirals rather than blades) are more efficient why are the standard wind turbines used?

Answer 1

the more solid the swept area the more torque, but the slower rpm.
ie. electric generation require high rpm, but low torque, so fewer & narrower blades reduce drag & spins faster.
Water pumps need high torque low speed & these have almost solid disk
Flettner rotors will generate more torque & so are mostly used for low speed propultion

not sure how you mean flettners are more efficent. the important factor for power is the swept area regardles of if the spiral is solid or segmented blades which form a spiral when spinning.
Theoretically optimum for generation would be one v narrow blade getting near supersonic tip speed - but there are construction issues with this

Answer 2

not sure i would guess either they are cheaper or more stable