Very good point. Which is why I think that it sometime's harder for someone to teach something if they're very naturally good at things. Some of the best teachers I've had or seen where people who found it tough to learn themselves.

Having said that, this is partially psychological, as it's more encouraging if the teacher can say that they had to work at this too: it means the ability/understanding seems achievable. Plenty of teachers pretend that they don't think something is easy or obvious to them for that precise reason.