The benefits of order flow are a little more subtle than that. Imagine a rising market and you have been hit on the bid. You immediately have the bid ask spread as a cushion (stop loss) for your long if you know there are other orders there to buy too.
Very. One can avoid stepping in front of a running train given the info i.e. Goldman just sold a ton of 9m10y, so I'd avoid getting long gamma, since they might have more to do. Also, a market maker would generally maintain a negative gamma bias. This way, in quiet times he's long theta, while in volatile time he'd be the first one to see the bids and lean his markets accordingly.