I disagree. Pasting plain text removes additional attributes, an operation that forces the user to redo work (add highlighting, bold, font choice, etc). For average users who will never learn about special alternate ways to paste, they NEVER want to redo work. They will hate it. For those of us who care, we can always just paste into a plaintext editor if we want plaintext.

Defaults often become "the only available option" and in that case I (and I think most folks) would prefer the defaulting to "as much info as possible is preserved on paste".