The junk goes to stderr, so it won't impact anything but your screen when used from the command line. And sometimes (large transfers) you want that information. I think the default is sane. When you're using curl in a script you use -s, when you're using it interactively you want the diagnostics.
Yes, sometimes when it's a particularly large transfer I want a progress meter but I normally don't so it's in the way; perhaps when we all had modems. And from a script I'd still want to use -sS instead of -s because although the script is obviously checking curl's exit value the user wants to see curl's specific complaint before the script's own knock-on diagnostic.
