I usually will never not have an introduction forum. If anything, I'd make an introduction thread in the off-topic section of my forum. However, I can see why some forums would not have one (specific niche and/or support forums). I send an automatic welcome PM and e-mail, though.
My e-mail/PM is nothing special - standard BS. I know some other people here have really well-written and customized ones, though. But they're important (the introduction section), I think, because they're naturally expected. Depending on the niche of your forum (and your forum does apply to it being important), people might be confused because they cannot find such a section which is what they look for as soon as they're done registering... only to find out there wasn't one. Not worded how I'd like it, but I'm very tired. Hope that made sense.
I use this features with well-written texts. I often get complimented for the welcoming pm/e-mail Until vB 3.7 I used an introduction forum. The renewed profiles of vB haven proven themselves to be much more effective. I believe the number of visitor messages are 10 times more of the number of posts in the introduction forum.
This is a built-in feature of vBulletin and you can enable or disable it. You can also change the title and content of the pm. I use welcome pm and a mod that posts a welcome in the new members' visitor messages in their profile. I feel that it is very important for a new member to feel welcomed and wanted on your forum. The welcome PM can serve as kind of a "getting you started" guide, pointing them to the intro section, then to the topics section, etc.
Because my topics include real life stuff, I have an introduction section and a connections section. Its recommended that if you want to meet people in your area, that you post a thread in the introduction section with your area, state, city, maybe a zip code,,,, that way others in the area can find you. The welcome email and pm that is sent to members is a mile long and contains links to important sections of the forum.
I do think introduction forums are necessary, because it offers a place for members to "break the ice" and ease their way into the community. Joining a group of people who appear to be tight-knit and close isn't exactly the easiest thing to do. Some people may feel unwanted or excluded. By introducing themselves and discovering how welcoming everybody is, those feelings of exclusion dissipate.
I find it helpful to suggest some things they can put in their introduction if they don't know what to say.
Personally, I feel that having an introductions forum is crucial. When a new member registers on a community, they can't help but feel somewhat intimidated - he/she is new to the environment and the memberbase is completely foreign. By including a place for these newly registered members to introduce themselves and/or make the community aware of their presence, you are assisting them in becoming an active and participating member. Go away, shyness - come out activity!
In my opinion, an introduction forum is imperative. As Nick brought up, it offers a place for members to break the ice. Not only that, but when new members join the community, the already common and regular members may want to know a bit more about them.
This echos my view exactly, members are always asking for new forums that I struggle to fill without creating more clutter and an introductions forum isn't worth it for me. I find a thread serves exactly the same purpose and even gets people talking more as their introductions are mixed in with the introductions of others.
If the clutter is really bothering you, I've seen Introduction forums combined with Off Topic or Announcements forums. Not sure how well that works, though.