Why is Script needed?
Why can't any transaction be simply "move X BTC from key K to key I signed by private key K"?


Then you couldn't (easily) do the P2SH stuff being implemented right now without getting everyone to upgrade. You also couldn't do escrow, multisig, secure cross-chain trading, secure high-frequency trading, etc. These features are very valuable; Script-based escrow, for example, will be the first escrow system ever which requires no trust in third-parties.

Script is, in my opinion, one of the greatest innovations in Bitcoin. I would prefer a much more powerful, Turing-complete version of Script in Bitcoin, actually.