The screen on the T3i is very sturdy. I mean, when you've got it sticking out, you gotta watch it, but it's just like being careful not to smack your lens into anything.
It's not a necessity, but it can come in quite handy. I was just out today taking some shots and wanted to get a shot down very close to the ground, but didn't want to lay down in the snow and mud. So I just flipped out the screen, tilted it to face up and held my camera very close to the ground. I could see the shot easily without needing to have my face at the viewfinder.
I've also used it while trying to shoot long exposure star and meteor shots. Having the camera pointed straight up on a tripod makes it difficult to look through the viewfinder. Flip the screen out, twist it however you need to view and you are good to go.
It actually can even provide a measure of protection for your LCD screen, because you can flip it completely around and close it again so your LCD screen faces into the camera body and is protected from scratches.
If you wanted, you could probably find lots of uses for it. Hold your camera above your head with the screen flipped down so you can get overhead shots of a crowd, or use the screen to take candid shots around a corner...just push your camera around the corner and you don't have to lean out!
Even if you end up not using it, it's not going to hurt you to have it. When you close it, it sits into an indent in the camera, so it is flush with the outer edge of the camera body. So it doesn't stick out to the point where it can get caught on something and knocked off. If you don't use it, you'll forget that it even flips out. In my opinion, then you at least have it for the times that it might come in handy, as long as you remember it.