Is generic "thank you" email a waste of time in academia? A very busy professor just responded my email. He answered my questions, and in the end, he explicitly said something like "Please let me know if you have any questions" or "Please respond if you have further questions".

So, I am trying to think as if I am in his shoe. Is he not expecting me to reply to this email, if I do not have any further questions? 

Is it impolite if I do not respond to this email and let it go? Or, is it considered impolite if I spam his "already-too-full" mailbox with a generic "Dear XXX, Thanks for XXXXX, Regards, XXXX" ?
If you send a thank-you note, in addition to being polite, you let the professor know that you received their email and don't need anything else from them. That lets them avoid trying to follow up with you later.