The organization can represent the customer and it can represent who the
employee or contractor works for. Provided these users are not creating the
projects, products, or libraries themselves, yes you should be able to use
them provided that you make sure the property to expose users of that
organization (checkbox on UI when creating an organization) is checked. If
these users do need to create projects, products, or libraries in other
organizations, then that is an access control rule. Typically this also
mean you set their domain of user to site/unaffiliated which happens by
default when you leave that field blank on a new user screen.
I use multi organization in my setup. It offers a lot of flexibility.