You don't need to switch interfaces to look up words on the Kindle. On my Kindle Keyboard, I can take the caret to just before a word for a quick definition popover. Press return from there to go to the detailed entry on the bundled Oxford Dictionary of English.

In Kindle Touch you touch and hold a word for a second or two but that's beside my point. I want the ability to tap a word or move the caret in the quick definition popover to look up another word within the same interface without having to switch to the full dictionary.