That would depend entirely on the app. I have donated many times and paid for apps, sometimes I just donate to the app even though I may not use it much but I like to support developers who support the user community. Like ROM developers, I always liked helping them.
Now a developer who offers a basic/lite app and an enhanced/pro version for a small fee well that is an app you really have to ask am I going to use the advanced features and if so then its worth donating. But again sometimes I just buy the app to support the community. I have purchased countless apps both Android (and iOS, and WindowsPhone, etc) just to help someone keep their passion for developing going.
Its not charity in either case, but supporting the community and getting something in return.