avsa

Your site can be taken offline either because it can't handle too much traffic, because authorities seized your servers or simply because you decided you didn't want to play anymore. If that doesn't happen and your site becomes very popular, you might be tempted to create a bunch of coins on your own mysql database to pay yourself.

Blockchain prevents all that: your code is committed and public

I used Mist as the title because it's an ethereum blog, but you had a good point when using it for other contexts

For now Mist requires a full blockchain but we already started testing a light-client on the node that would avoid that.

Not all app need to show ether to the user, but that is the currency used to pay miners for transaction fees. There are ways in which the app developer can try to abstract the concept of ether using other currency, or even subsidising the users by paying their cost via other means (like what digix gold does).

Ether is not used as a reward for hosting the app, but for changing saving data and changing states on the blockchain. We will have a P2P solution where users are paid to host actual application files.