It's cheap. You can send a large amount of bitcoins anywhere in the world with very very low transaction fees. Merchants can accept it without a ridiculous fee.

If you convert it from/to cash immediately before/after each transaction, then you face exchange fees. And if I understand correctly, most exchanges have daily limits. And anyway exchanges are shady businesses: see what happened to MtGox.

If you hold significant amounts of bitcoin then you face volatility risk.

While your funds are stationary they are not subject to seizure.

Why not? I'm pretty sure that your government can force you to hand out your wallet.