You don't necessarily have to buy something you don't want. Steam has a huge library of games (even games under $5) and a variety of general software, creative software, and so much more. Most of which are at fair prices. Not to mention, anything you buy through Steam, you will easily be able to re-install if you change computers or have trouble with your computer. If all you have to do is spend $5, what's the concern? Chances are that Planet Coaster isn't the only thing you will get from Steam.
In this digital high-speed world we live in, more and more companies are releasing their computer software or games through platforms like Steam, Origin, Ubisoft, GOG, etc. For those companies, It saves costs on retail space, packaging and of course shipping, maybe even server-load. I really don't see why this is going to cause a problem.