If you're day-trading, it's faster to stay on an online wallet. If your coins are offline, you may have to wait anywhere from an hour to more than a day for confirmations before you can use them online. When Litecoin prices started climbing today, I had to quickly get my Litecoin client up... resynced and send some coins to btc-e.com and wait for _six_ confirmations on the blockchain before I could sell/trade them. I'm just lucky I didn't miss the spike.

That said, I'd rather risk missing out on peaks & valleys than suddenly losing thousands due to these kinda shenanigans.