Doesn't necessary, but could be very useful
I find it useful for customer support. Indeed, discussion is needed when site support is weak and there is a chance to get help from the community. Also community could be more trusted source than internal support.
Hovewer, context, type of goods, and overal experience should be considered. For technical goods it's just fine. Also I moderated the site which sold women's apparel. Probably women are more critical to apparel's details and they sent a lot of questions about goods.
Some examples of how discussion works on tech sites:
- Weggle – they sell bike components. The internal support is fine, hovewer it's useful to get answers from other customer, as they could disclouse some important issues and share real experience.
- DX – sells cheap electronics. Customer support is rather weak, moreover goods could be slightly changed compared to description. In this case discussion allows to share experience of previous buyers. And really valuable answers could be found there!
Ofcourse, it comes as addition to other tools, like customer reviews, FAQ, etc.