Many restaurant websites actually put an OpenTable link on their reservations page, directing traffic there.

That makes me think the restaurant prefers reservations to come in that way. They don't just see it as a secondary reservation system to bring in more customers, but the primary channel.

The added efficiency of OpenTable cannot be underestimated. Especially when trying to book something for a large group or at the last minute, being able to see availability across many restaurants in a single glance is critical. I'm not going to bother calling.