Yes, they absolutely are. Hell just getting a few random bots scraping stuck in a loop or being overly aggressive on your site is enough to double your bill. So yeah it's 100% required.
One bot that's just stuck on a loop or being overly aggressive is going to have one IP.
Do you pay a variable amount for your hosting? How and why? All VDS and dedicated server offerings I've ever seen are fixed amount per month. And more often than not the network is limited by speed, not by data transfer.
Mind you, I see your point and I generally don't like the captchas either, but it is definitely a trade-off and I won't blame webmasters that use the DDoS protection.