If I were doing Light Table, I'd use GPL too - It also does not preclude someone from writing their own plugins and selling them as an add-on. I would assume there will be a way for easy/seamless plugin installation into the main program.
No one is taking it as one, but poe's law aside I'm convinced they are wrong.
I don't get it... is your added value simply an illusion?
If they can see what you've changed the code, they should be able to see why and how you added value.
It will be harder to convince customers you've added value if they can see exactly what you've changed.
Sprinkling the GPL on top of a project no one wants anyway does nothing (I say this from experience), and applying it to a popular program just holds it back (by keeping it from getting the cash that people would otherwise pay).
As an end-user, I want to see Light Table take off, and to continue innovating, not to continually engage in campaigns for donations.