Respectful for guests, too
Start any relationship on the right foot. Putting all the scheduling legwork on your potential customer makes them feel like they're not a priority for you. Same with asking them to sign up for a service they don't want.
Boomerang scheduling works inside email, so it respects your guest and saves them time and effort too. They don't have to change their workflow, leave their email, or reply to the fake robot assistant 8 times before meeting with you.