There are a few PAAS providers that now provide a free tier of service and cheap prices afterwards. Some, such as AppFog and OpenShift, also offer easy installations of common web apps such as Wordpress. Is there any reason to still use a standard web host for hosting things like PHP scripts or Ruby on Rails apps?
Is there any reason to complicate your life using AppFog/PhpFog spendng 29$ per month, when you could simply buy a 45$ per year reliable cPanel hosting service with MySQL (among millions of providers) that will suffice to most of your web application needs for a long time?!
And in the case you have a lot of traffic you simply need to send an email to the hoster and ask them to move your plan to a bigger one, or to a VPS still with cPanel. Even for VPS plans you can find something cheaper of PhpFog.
Last but not least, if you are unhappy with your hoster you can move to another one (there are millions online providing cPanel), with strange stuff like AppFog/PhpFog you risk in the worst case to remain stuck there, and in the best case you'll anyway have fewer alternatives vendors to choose among.