If a product is offered for free under the GPL license than why should I donate to the product developer?
I can get this for free, use it, modify it without giving a dime. More than that, if I donate, this will not give me any extra guarantee for the product and I will not get some special support than to other users of this product whom are using it without donating. So why should i really donate?
Well, in part YOU are right, but lets just think about it for a second. Product developers invest time and resources in order to release a quality product. This product can be commercial and only a few some whom are willing to pay will have access to it, or can be released for free and everyone will be able to use it.
The beauty of donating is that it gives the possibility for those who cannot afford to buy a product to use it for free and makes the developers think about releasing other products as well in the same manner. This can only be achieved if those who can and afford will donate and think about the advantage that not only YOU will make others happy, but YOU decide how much this product worth after you tested it. Think about how many times after you bought something turned out that is not really exactly what you wanted.
If you enjoy using quality products for free and those products will help you in whatever you are doing, think about donating and keeping development of free products alive.