There's an excellent reason why you've never seen seafood stock made from clam shells. It's a totally worthless proposition. What you'd basically end up with is, as others have mentioned, the equivalent of water simmered with rocks - but with a bit of salt, perhaps.
And DramaQueen - you're not reading for interpretation. No one said the clam shells were bad for you, but there's a HUGE difference between enjoying clam broth made from steaming whole clams & boiling the empty clam shells thinking you'll end up with an enjoyable broth. There is absolutely nothing to be gained from adding empty clam shells to anything.