Certain wallets may be able to use only the output script filter by using output scripts to watch for confirmations on sent transactions, assuming that application is the only one with access to the private keys. The additional benefit of the input script/outpoint filter is to watch for unexpected spends (coins getting stolen or spent from another wallet) or transactions without a unique change or output address. I think this is a reasonable implementation, and it would be nice to be able to download that filter without any input elements.