The value of the GPDA is one issue; but Felipe choosing to leave it is another. One wonders what's happened to bring him to the decision. There doesn't appear to be any comments from others about the way it's being run; although there was some debate on my forum recently about drivers who aren't members - Kimi in particular.
At the end of the day, those that aren't in the GPDA won't have a say, and the decisions made by the GPDA do directly affect their safety. It's very well not being in the GPDA when there's no issue - and another when theres the occasional safety concern (such as concern about painted areas of tarmac etc etc that we've had in the past). Then those that aren't in the GPDA won't be able to have their opinion included in the general view.
I don't think its necessary for every driver to be in the GPDA, but I think there should be at least ONE from each team, so personally I'm disappointed that both Ferrari drivers are now not included, particularly when Schumacher was so supportive and instrumental within the GPDA in the past.