Instantly? I thought the transactions took an average 10min. Am I wrong?
The issue is that you might send multiple such transactions spending the same money. If I want to guard against others trying to spend the same money I have to wait until the transaction is a few blocks deep. Generally the advice is 6 blocks which should be about an hour.
A bigger problem right now is that you pay whatever the current rate is. With the current volatility you could easily overspend or underspend by a large amount.
Steam is no longer supporting Bitcoin | https://news.ycombinator.com/item?id=15863182
> due to high fees and volatility in the value of Bitcoin
Even 0 blocks is somewhat safe, as long as you keep resubmitting the transaction and the other person doesn't spend those funds in an earlier block before yours gets included.
https://smsprivacy.org/zeroconf-payments
> We've decided to start accepting 0-confirmation payments under certain conditions:
> 1. The payment amount does not exceed 0.02 BTC.
> 2. The total amount of pending unconfirmed payments across all users does not exceed 0.1 BTC.