Hi,
I think that is actually no question of IF Content Management System or not, its a question of which CMS fits your clients needs best. Most clients will come back anyway - these days there is no way a client can succeed by having a website prepared once and then just left with him, they will expand the features and they will have to integrate it into their marketing efforts as well.
so its part of the responsibility of the service provider to educate and consult the client in that matter. Whether its Drupal or any other CMS is a different story [I prefer Drupal, though]. But you simply canīt serve the clients needs on a sustainable level if you just use some sort of static content, except for some very very small clients who just wanna biz card online basically.
Developing a CMS oneself - well, that is in my opinion a complete misallocation of resources - there are tons of service providers you can use and Drupal for example is not that steep of a learning curve that you would invent the wheel again. the other thing with proprietary CMS - how will you ever be able to provide service level agreements to larger clients - they require that, but you simply wonīt be able to serve them. Unless you have quite a lot of equity in your hands to build up a structure which can do that - but i guess at that point you wouldnīt really think anymore of whether or not CMS, but which one.