The most important part of the user experience that multi touch input enables might just be an onscreen keyboard that doesn't suck by being frustratingly slow and/or error prone. Requiring the user to only ever be touching the screen location of one key at a time slows the user down. And when the inevitable happens, i.e. the user is touching two on-screen keys at once, to take the average (which lots of older single-touch screens did) is not acceptable for text entry.