I don't think it would be any use. Many members wouldn't keep it up-to-date unless it was automatic, which there is no easy way this could be done - just because you're a licensed user doesn't mean that you are using the latest version (or the latest version you are entitled to), so how exactly would an automatic system determine what version you are using without it directly checking your registered website (you can't assume that just because someone has downloaded a version that they are using it)? That would be a whole lot of effort on the part of the Xenforo team to get this working and for what benefit exactly? So a few people can look at other people's profiles to see what XF version they are using. Definitely not worth it!
Nice thought - but I think, on TAZ it has such a field and I filled it in at the start and have never updated it - so I'd guess the same might happen here.
Also I think here people generally adopt the cutting edge builds pretty quickly - which I goes against the norm for a PHP app (it's expected more in Ruby circles - one of the huge differences between it and Python for example).