> Is there an application that requires watching for output scripts that
> doesn't also require watching for input scrips (or, less efficiently,
> input outpoints)?
>

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.