Casual is meant to be casual. Women wear theirs out and so men should be able to do so, as well.

Unless the shirt is extremely long

Tucked is like having a dog on a leash, all constrained and uncomfortable. Untucked is like a cat, free to waft in the breeze and remain relaxed.

It's More Comfortable Looking

Tucked in is so strange looking

If it's casual, be comfortable!

This actually depends on the build of the person. But if it is casual, untucked is casual and comfortable.

It’s in style

It's more comfortable this way.

I like the casual look of an untucked shirt.

I think if it’s a casual shirt it just looks better untucked.

Tucked is upright and not fashionable. Shirts come is so many different cuts that you should be able to find one that does not require tucking.

Their "Done-lapped" disease is not as obvious with the shirt untucked.

More relaxed look.