Many of those I know are consultants or freelancers, i.e. there's no employer to cover the expenses and no revenue is generated while they're doing speaking gigs.
While the marketing value of being a high-profile, sought-after speaker and expert certainly can't be underestimated, when speaking at a conference most of the time you aren't exactly speaking in front of your target audience in terms of marketing but in front of fellow designers, developers, you name it ...
Giving a talk at an event certainly is a great personal experience and a tremendous personal growth opportunity. However, I can't help but wonder if for a self-employed consultant regularly doing speaking gigs really pays off business-wise. Perhaps someone with personal experience in that area could shed some light on that?