Since this was my suggestion I will lobby for it publicly here.
First, I feel that an introduction thread is quite distinct from Profile info - the latter is for people who have already joined. The former helps attract people who are on the fence and perhaps lurking already. Profile info doesn't help grow the user-base. An Introduction thread might.
I help run BoardGameGeek (specifically I head up the sister-site RPGGeek.com) where we have just passed a half-million registered board users. Over the 10 years there we've learned quite few things about attracting new people. Without new people, hobbies languish and wither. I can, from experience, say that the forums here at Reach Mahjong are well structured and the site is well maintained but jumping into discussion here is intimidating. Several times I've wanted to create an account and post a reply but didn't as I felt just a bit too 'noobish'. Even the Dojo can find some heated discussion (and asking questions when you're already a little new and gun-shy doesn't feel overly welcoming). I've run across thousands over on the Geek who felt the way I do. It's not easy to jump in with both feet first. Heck, it's hard enough just dipping your toe in the water!
An introduction thread is just that - a place where you can make your first post and give a little background. There are no expectations as to what someone might post. Even a simple 'Hi. I've been lurking for some time and finally decided to get an account.' might be enough to get them over the hump. Board regulars generally watch this thread and jump in to welcome new folks. This gives a very safe and clearly delineated area for an entry-level post that doesn't require the person to have a specific question or respond to an existing discussion. It's a thread to get their feet wet - hopefully they find the water warm and stay.
The current forums are like a group of close friends who are standing in a circle talking. You can listen and you can even jump in but it can be intimidating to just up and start discussing with them. Having a warm friendly thread will seem inviting - witnessing all of the other newbies who joined can create a bit of a 'snowball' effect. At worst, it brings no new people to the discussion. However, even if it only brings a few people in from lurking status, I suspect it will have been worthwhile.
As an aside, I'll fill in some of the proposed optional extra profile info here: Username usually DaveB or wavemotion, male, blue... no red!, played since 2006 (video) and 2012 (live), B-Cup, Lawful Good.