At the very worst, the inverse of familiarity breeds contempt.
But in any case, either misery loves company, or you are incorrect in your assessment of its codebase relative to the alternatives, given that vBulletin has "thousands of clients, including many industry leading blue chip companies".
Are you inferring that popularity and familiarity are not pivotal in ensuring good traffic? Are you suggesting that novelty for and of its own sake has merit in this context?
vBulletin is far more than "just familiar to you as a typical forum look" it contains much more relevant data on a per area basis, it's collated in a much more logical manner and there is much less wasted space on a per area basis.