If you look at Electrum/b.i/Mycelium/etc then the main feature they add on top of that is the ability to query a full block chain index rather than scan it. This results in faster query response times, but at the cost of maintaining a huge database on the server side.
btcd maintains a full blockchain index by default. I think Bits of Proof does as well. Not sure about Obelisk.
Bitcoind may in fact be the odd implementation out by not doing that by default.