I've actually done some side by side tests with stains using conditioner vs not. I wouldn't call it necessary by any means. Depends on the stain and the wood but sometimes I did feel the result was noticeably better, sometimes I couldn't tell the difference.
Parent commenter might specifically be talking about yellow/white pine, which is notorious for splotchy staining and yellowing over time, which conditioner can mitigate a bit.
If you're working with oaks or hardwoods, I agree with you.
