I don’t think anyone here will debate that, but it’s a much more polite way to reject someone than telling them exactly what’s wrong with them and that you never want to see them again. Even if the rejectee really wanted to hear it–and I strongly suspect that they’re lying if they say they do, considering my own experience with the direct approach to rejecting women I wasn’t into, and considering how difficult it is to take something like that anyway–then one should take the feelings of the rejector into consideration. Nobody wants to say that to someone, unless they’re a really mean person and/or they’ve had a really bad day–and even in that case, they’ll probably feel like an asshole for saying it and dwell on it for a while. Saying you have plans and maybe you’ll call back is the polite way to turn down any invitation, including an invitation to date someone.
A rejector just can’t win, anyway–I remember an ordeal with a woman who just wouldn’t take no for an answer, and first I would reject her gently and politely and she wouldn’t get it (or she would whine about how I was being evasive and I needed to be specific or some such crap); and then when I put my foot down and told her that I never wanted to see her again and that she should consider herself rejected, she would leave voice mail messages and call me from blocked numbers to tell me how mean I was and by the way did I want to fuck sometime?
Being told something like that is deeply embarrassing, as is the act of telling someone that. Worse, people given a specific reason for being rejected tend to clamor back in a short time and say, “I’ve changed all the things you didn’t like! Please go out with me now/take me back/whatever!” Which is generally a lie or a delusion anyway, isn’t a healthy attitude, wastes both peoples’ time and is extra-creepy too.