But no seriously, even a one-word response shows that they read and acknowledged your last message, so i personally don't really see that as inappropriate in any way, unless you can give me a convincing example otherwise.

When two people who love to get the last word in are saying bye on the phone, now that's amusing. But I think everybody does it to some extent, the "Ok thanks so much, speak to you again soon." "likewise! have a nice day." "Thanks! Ok bye now." "Ok bye, take care". "You too!" "Thanks!"

I wonder how long this is allowed to go on before it's socially unacceptable, and likewise, how short is too short? I know one man who you'd be in the middle of a conversation with, and as soon as he's decided he's done with you he says "bye now." *click*.