I only consider purchasing if there is a huge difference between free and paid or if adds are unbearable. its very rear for me to purchase a donate version because I feel that its worth thanking the developer for making the app.
I like to support the developers for the apps they make. A lot do it as a second job. Especially if it's an app I use all the time. GoodReader for example. Probably one of the best things I've spent $5 dollars on. I want developers to make money.
I started out in the smartphone thing thinking I will never buy, then I built and sold themes, now I agree with @BHP41 and try to support the devs when I can. If there is a pro version generally I will get it. Yes Roger you are right to get the max out of the app you generally need the paid version