The new system doesn't require that you use the site for everything, if you just plan on using a forum topic that is fine too. However the new project system allows you to have your screenshots organized together better, if you post an update through the system it shows up on the forum in the Project Updates board (i.e., more forum visibility for your project), if your project gets selected for the Project of the Month it will later allow us to have it on the site's main page, the site allows you to upload project downloadables, and it allows other team members of your project to update the sites information. Additionally instead of having a comments part of the site and a forum topic for a project, the forum topic is now considered the site's comment section.
Some of the projects in the old system will have to be manually transfered over because their creators are no longer active or just don't care - but the site pages can be edited to allow them access to their created pages. Oh and yeah, I've had issues with editing too sometimes, when all else fails I make a manual edit to the database.