Firstbits is essentially a scheme for registering a short name for a static bitcoin address. So far so good, but it uses the blockchain to store the registrations. This encourages spamming the blockchain to be the first to use a particular prefix, and is not compatible with pruning (consuming the outputs of the first transaction that used a particular address, may change the firstbits mapping). This problem can be avoided by keeping a separate database of each prefix along forever, but at that point it has no advantages over a third party registration system for address aliases anymore.
Furthermore, it enforces the notion that a single static address should be tied to an identity, instead of using a new address for each transaction. Almost by definition, since a firstbits mapping is intended to last forever.
For these reasons, I would not accept such a patch for the reference client.
You need a full chain for firstbits lookups, you can use a pruned chain if you are just mining. This is another good reason not to include it in the main client.