I think it's funny that people think that having a large vocabulary makes you intelligent.
I would have thought that having a mastery of a smaller vocabulary would be altogether more 'intelligent' given that it increases the chances that you'll be able to communicate (not that you actually can ;) with a greater proportion of people. (particularly if the 'smaller' vocabulary is common to a greater proportion of people, as is typically the case with words that might be referred to as 'common', and assuming that communicating with a greater number of people is 'intelligent' ;). Further, as a human you have finite time. During this time if you are able to sufficiently communicate with others with a limited vocabulary, then isn't it more intelligent not to *waste your time* learning to use words that others are not likely to understand..?
Being intelligent, and being well educated are two different things.
I think mostly it's education that is important. I don't think that any given person is particularly any more or less capable (i.e. intelligent) than another. I think it's just that they haven't had their neural network sufficiently trained with the 'right' patterns.
As usual, it probably just boils down to a social status thing. Which in turn probably boils down to a mating thing.