I wouldn't implement accounts/purses at a low level.
Bitcoin-Qt did this and now we still have a weird and insecure
accounting system coupled with the basic bitcoind API.
People think it does things that doesn't do. We even had an exchange
using these "features" for the accounting of the exchange in
Freicoin!!
We have disabled those JSON calls in freicoind and I hope they will
eventually disappear from bitcoind too when the wallet and the core
are properly decoupled.

--
Jorge Timón

http://freico.in/