I have to ask because I have been noticing the pure number of people worrying about having their fermenter locked down so tight I'm waiting for some one to come up with a lock and key system.
1) Sure the lid has to be secure. This will keep it on if something hits it. But, I've had the airlock pop out of a carboy with the stopper before and it didn't seem to matter how tight it was.
2) The fermentation process creates a layer of co2 on top of the beer. This is a natural protection layer for the beer, keeping other gases out of your beer, but it's not a force field and won't keep anything from passing through it, like a childs toy. The reason for a lid on a bucket. I've used both buckets and carboys. When I use a bucket, I lock 4 of the "tabs" on the lid opposed to each other and call it good. I have yet to have an infection in any of my beers (knocking on wood at this time) or any other real problems.
Do some research on open fermenters, they are as open as the name suggest. On a tour of a brewery in Augsburg Germany, they had many. In a room with several open vats and extremely limited air circulation. We went through an enclosed hallway to see it.
I just think it's funny how people post pictures of a violant fermentation popping the lid off a bucket or blowing the stopper out of a carboy and keep trying to find a way to get the lid on tighter. If you have such an active fermentation that it blew the lid off your bucket, rejoice. It just means your yeast was really good and very alive. Then sanitize the lid and put it back on so nothing (physically) falls into it.