Well, it's not *entirely* true that it has only backwards compatibility to recommend it: without -c in its current form, there would be no way to test multiple-commands-in-one-PQexec cases without hacking up some custom test infrastructure. That's not a very strong reason maybe, but it's a reason. And backwards compatibility is usually a strong argument around here anyway.
I've not been following this thread in any detail, but have we considered the approach of allowing multiple -c and saying that each -c is a separate PQexec (or backslash command)? So the semantics of one -c wouldn't change, but commands submitted through multiple -c switches would behave relatively unsurprisingly, and we wouldn't need two kinds of switch.
I believe it can work, but there are stronger limit of single PQexec call - only result of last command is displayed.
Regards
Pavel
Another issue here is how -1 ought to interact with multiple -c.