Alice gets her computer hacked by Mallory, and she steals her wallet. Fortunately, Alice had encrypted it with a strong passphrase. Alice wants to sue Mallory for breaking into her computer, so she gets a lawyer, Bob.
- Alice's address, 10BTC --> Bob's address
Unfortunately for Alice, Mallory controls a node on the Bitcoin network, not too far from Alice, that quickly sees the transaction is from Alice (she has all her addresses), and instead relays, to hundreds of nodes at once, the transaction:
- Mallory's address, 1000 satoshis --> Bob's address
Which is incompatible with Alice's honest transaction.
Now, three things happen:
1) Alice tries to send the bitcoins to a different address given by Bob
2) Alice's nodes see her transaction as a double spend, and refuse to relay any transaction spending that coin to a different address
3) Mallory's transaction eventually gets mined. Back to trying to spend the coins to Bob.
Mallory can do other mischievous things with her position, such as allowing Alice to spend her bitcoin at a church fundraiser but not an abortion clinic (assuming she knows the address of both).
The main way people lose their privacy in bitcoin today is through heuristic analysis of the blockchain, not address reuse. Introducing a new network rule to protect users against their own stupidity will not significantly improve privacy (most wallets don't reuse addresses by default anyway), and opens up users to some interesting attacks.