I use Thunderbird as my email client. If you turn vCards ON it seems to send every email with a vCard, which is not really what I want. Since then I updated my signature and put most of the information underneath. To be fair the vCard facility in Thunderbird is somewhat poor, maybe I can look into a Plug in for Thunderbird, I am sure they will be something out there.
I have never really sent a vCard, or been sent one, so I can’t say I know much about them from personal experience. I remember once I had all my contacts in my address book, then my computer got a virus and so did my contacts. I never saw them again.
I will probably send them initially to all my contacts one and then every time I update my details.
I think it depends on your audience (in my experience I know people who have found them useful). Though I would say “why not” is probably the better answer than “why”. After all, vCard’s are actually VERY easy to create and therefore having one available for those who know how to use the things will avoid them having to manually enter the data. While I understand your scepticism as to how useful they will be, the best way to go about any practice is to weigh the cost of implementation against the benefits it will have. In this case, I really don’t see any justifiable reason not to offer such a potentially useful time-saving file.