This is all generally true, but there are times when a player you are going to send down may have value in the future:
1) You need an extra arm in the pen (overestimated the innings your guys could give you). So you send down your last pp, hoping to bring him back up after roster expansion, or you send down a low sta/dur guy from the pen for a better sta/dur guy with worse stuff, again, hoping to bring him back later.
2) You make a deadline trade for a star-quality rent-a-player to get over the hump in your division, and need to demote an okay ML player to make room on the roster. He may not be your worst ML pp, but because of roster composition he may be the guy who has to be sent down.
3) You are lucky enough to find a pretty solid LRB on waivers, who is pretty much a wash with your current LRB, only the new guy is out of options, and your current guy isn't, so you do what you have to do.
In these sorts of situations, I would worry about the demotion hit (perhaps demote the guy with better makeup if I had a choice). Probably rare (once a season, maybe), but it happens.