The reason for categorizing open house guests within specific groups is to allow you and your staff to focus on the business at hand. The approach you'd take in dealing with a specifier would be somewhat different than the tack you'd follow with a trade mechanic. Both are valuable to your business, so you want to be sure to treat each just right.
You may also want to have door prize drawings at your open house and, by separating your targeted groups, you can be sure that the prizes you furnish will be specifically suited to the background of the recipients - be they designers, specifiers, installers, or consumers. Scheduling logistics represent another reason to separate the groups. A breakfast for contractors, a wine-and-cheese cocktail party after work for specifiers, and punch and cookies for the public on a Saturday afternoon would be good ways to strategically target and entertain these diverse groups.