A Lens Is Delicate and Fragile...
It just makes sense to treat a camera lens carefully.
A telephoto lens is expensive.
A lens is composed of layers of precision-ground glass.
A good, sharp lens helps you take better photos.
You don't ever drop a lens.
Or handle it roughly.
Never mistreat it.
The lens should be in a protective case.
Use the lens cap.
Keep a lens dry.
Of course, if it is a coffee mug that is just designed to LOOK like a lens, then you have more leeway.
You can make tea in it.
My Canon S90 camera is a Point and Shoot, a P/S, and it does not accept interchangable lenses.
Or coffee mugs.
In fact, if I had to carry around different lenses and filters in a big gadget bag and a large tripod, it would be a much larger camera - probably an DSLR.
Then I would miss lots of pictures because it would be bulky to carry. And heavy. And I would be lazy.
I would probably NOT have it with me when a great photo opportunity presented itself. Yikes.
It IS fun to imagine having a great big lens though.
(I hope people actually read this and not just shake their head seeing the making-tea-in-a-lens picture. They would think I'm a dope.)