I visited Yellowstone NP recently (my first time, beautiful country.)

I'm not much of a trout guy, but my buddy and I decided to take a drift boat trip. We were throwing a lot of dry flies, and my friend started using aerial mends (reach casts) to defeat drag. Our guide immediately corrected him, though. He was adamant about not using any aerial mends. He told him to let the line hit the water and then mend.

What's the problem with aerial mends? Is the other way really better?