There are a lot of pros and cons to having an online community associated with your site. The cons mostly center around having to manage people within the community (dealing with flamers, etc.). But the biggest pro, in my mind, is that community begets community. If you manage to get even a handful of core community members who are passionate about your site, they will inevitably spawn new members as they tell others about it. And much of the time, their enthusiasm for the site will filter down to the new members, causing them to be enthusiastic as well. It is this growth that will make your seo efforts easier, because you now have people who will link to your site if they have sites, or will create buzz in areas that contain people who do have sites.
Communities can be established in various ways. Of course, you can always create an old-fashioned forum or chat room. No matter how popular social networks become, I find that forums still provide a great community atmosphere. But if you want to escape from the "community 1.0", you can always find new, more innovative ways to bring a group of people together.
Look at what SEOmoz has done with their community. They don't have a forum or chat room, but no one can deny that they've managed to create a place for their followers to gather and communicate. I don't even know what to call the application that is being used to allow the communication. It is largely based on blog comments, but it goes beyond that with some custom programming. I guess we'll call it a "custom blog community".
Obviously, creating a Pligg-based social network is another possible way of drawing a community together. Sphinn has done a great job of showing how well this method can work. And of course, external groups can be formed via Facebook for example, so that you gather your community outside of your site, but the group is still connected because of your site.
If you get creative, you can probably come up with many other ways to create a community around your site. One method may be better than another for your users, but the takeaway here is not which method to choose, but that you should create a community sooner rather than later. Regardless of how you choose to bring your users together, the act of doing so will likely bring about a lot of positive change for the site. As time goes by, you'll watch the community grow (even with no promotion on your part), and you'll see the positive results of community begetting community.