On talking to some people in #bitcoin-dev, it seems like firstbits, while useful for minimally referencing a bitcoin address, should not be encouraged, as it makes it much easier to scam.
I think this is a very real problem with the whole "vanity" address concept. Unless the vanity address is extremely expensive to create due to the number of fixed leading characters it will be just as easy for anyone to create their own address with the same prefix (unless the service will never list another similar vanity address in which case you could still be scammed by the service).