You obviously haven't fully grasped the potential of K2. Replying to what you said:
"You can only use K2. So if you decide to change systems or you already have a site built you will have to do it all over again."
This goes for Virtuemart too. If you build a catalog because it's more powerful compared to joomla's articles and then expect to rebuild the same with joomla articles on another site, you don't understand or don't know how to plan your work.
"The forum dosen't get much support." Not true. It's very vibrant especially compared to other more established components that are been around for more than a year (compared to K2's 5 month life). People have understood the benefits and now you can find any help you want in the forum.
"It also has user groups but you cant restrict the user groups to certain categories. Then what is the point of user groups?" User groups are for restricting content editing to specific categories and more. In v2.3 we plan to add an option to restrict content viewing, thus enabling its full ACL potential.
"If it integrated into the core then I would use it because it has many of the features I need." You cannot integrate such an application with the joomla core article system. You don't need to. K2 is as standalone as com_content is. It just has one menu item in the components and not a whole set of menu links in the backend and this is what confuses people that com_content is absolutely needed, but it's not.