Unconfirmed transactions are many orders of magnitude safer than "taking someone's word for it." The transaction wouldn't have propagated through the network if it wasn't valid, and it will be included in the next block regardless of whether the sender wants to revoke it. If the seller can see the transaction on their screen, then it's currently not possible to reverse it without a multi-milllion dollar double-spend attack or something very creative.
I believe, the risk of chargebacks on credit cards are much higher than the risk of an unconfirmed-but-valid transaction not eventually entering the blockchain.
Nobody has any proof that the transaction will be included in 'the next block' just because the vendor has seen it on the network. Nobody knows how the network will work when bitcoin is big enough to be used in point of sale. It's quite possible that some transactions will never even make it to every miner, so I can easily send out two conflicting transactions and nobody could be certain that they, or any particular miner, would see both transactions, and could not prove which one would end up in the next block; or even if there will be a block chain fork resulting from my two transactions that take a few blocks to resolve.
I don't know why vendors will expose themselves to the risks of a system that everyone can attempt to game without any negative consequence to themselves since they are anonymous. Everyone will just use clients that constantly try to double spend and get away with it; what's the harm in trying? Every once in a while you'll get lucky and get free goods without any chance of being identified as the perpetrator of the fraud.
You may believe that the risk of chargebacks on credit cards are higher, but until this is proven in practice nobody will know for sure.