Let me make this clear: When I paste, I want the text to match the document it’s going into. This is why I’d prefer Paste Special / Plain Text to be the default.
So why does Paste Special / Plain Text keep disappearing out of my corporate copy of Outlook 2003? If I’m stuck with rich formatted emails, I damn well want whatever I paste into them to match the text.
But every app wants to bring its own format across. Even Visual Studio for heaven’s sake. I end up having to paste it into Notepad, then copy/paste to the destination.
There are a couple of ways of making plain text pasting the default: you can define a macro in Word or here’s a crankily-named Outlook macro. If you’re aiming to get it everywhere, it looks like you’ll require a program: either Cliptext or PureText would appear to do it (in slightly different ways).