Besides spam vulnerability, free transactions raise another key concern. How are block producers motivated to keep producing? This is where EOS's 5% annual inflation rate comes in. There is no doubt that a large portion of the inflation will go towards paying block producers. As such, instead of explicitly paying fees to use dApps, which is a bad upfront user experience, holders of EOS are implicitly paying fees through inflation. Keep in mind that a 5% annual inflation rate is very aggressive. Even the US dollar does not officially inflate as fast (the official annual inflation rate is 2%).