[MPDev]

That is true in some cases - but some people don't have alot of time to rewrite code to their liking. Others like the option of support and installation services.

Here's an example of a community with integrate Classifieds, Photo Post and Product Reviews:

http://www.viperalley.com/classifieds/index.php (Classifieds)
http://www.viperalley.com/gallery/index.php (Photo Gallery)

Using a product like ours allows you to integrate with your existing community. PhotoPost 4.8 (in testing) and Classifieds 1.2 (both shown above) use vB's user selected styles:

http://www.photopostdev.com/photopost/index.php (vb3 integration)
http://www.photopostdev.com/classifieds/index.php (vb3 integration)
http://www.photopostdev.com/reviews/index.php (ReviewPost, about to be upgraded with new portal and style upgrades)

By maintaing a similiar look and feel across your features (coupled with the integrated user system), your users will feel comfortable in any of the areas on your site.

I'm not knocking open source, I've used it. More often than not it was a good starting point as I'd spend many hours customizing it to my site. Eventually I'd outgrow it and look for something else or, in my case, rewrite the application to suit my needs.

Always research your options - it can save you alot of time in the long run.