I'm actually more at ease with this change than if there was no default text. There's that flicker of recognition when I look at the input area before I start typing—"I'm messaging @channel." Call me paranoid, but I often flick over to the sidebar to see which channel I'm in before I dive into some conversations, just to make sure I'm not saying something in a public channel I shouldn't—like scanning over the recipients before you send an email. This proactively reassures me so I don't have to flit about making sure I'm doing the right thing.