We are looking the gift horse in the mouth. It's free.
Yes, you will hit the limitations of the free plan very quickly indeed, at which point you should start paying.
The idea of these free plans is to let tire-kickers try out the system. They don't have to pay to test drive the UX and deployment mechanisms, which are the value-added features that sell a platform like Appfog. They don't have to pay for low-traffic prototypes. They can launch a whole set of minimum-viable sites on free hardware and then upscale one within a few minutes of any traffic spike (in theory, anyway). And it empowers marketing maneuvers like getting an entire programming class of 25 people using your platform for a one-day class.
(I'm not sure I love the above business plan, to say the least, but that's the idea, and thanks to Heroku it has become the standard.)
I think that that's a good description overall, although I would add that the free plan allows deployment to all currently available IaaS and hence to do things like run analytics on the different infras to see what works best. But I would push back a little bit by saying that you can run apps getting some pretty legit traffic for free. It needn't be just "Hello world" or sandbox apps.
