@luke-jr 
There are some technical deficiencies in light bitcoin clients though it may be helpful for them with low download speed. But before anyone use it, they need to be acknowledged about the risk with bitcoinj and picocoin.
BitcoinJ and picocoin supports SPV(Single Payment Verification) storing only the merkle root hash in the only downloaded block headers chain.So it is a thin client(headers only client). And it will fail under a dishonest or powerful ISP(internet service provider). Chances are , it will be subjected to double spend attack with the increase of popularity of bitcoin. See details about the security issue in https://en.bitcoin.it/wiki/Thin_Client_Security#Simplified_Payment_Verification_.28SPV.29
On the other hand, Bitcoin Qt is a thick client(full) which downloads a copy of the entire chain, including all transactions (not just headers). So, bitcoinj doesn't really do all the things that bitcoin qt does,bitcoin qt ensures security.
However, there can be ways to improve I don't know for thin clients. But I think lots of people don't know about the risk that is not safe . ;)