Mining is a job that people can earn money by doing. People get involved to earn money and you are buying their support.
Since the currency is owned by all participants, no one is buying their support, they mine, they own, they keep, they sell. Even if some miners eventually sell all the coins they mine, their contribution to the ecosystem is no less important than someone who just buy their coins from an exchange.
People don't like 'handouts' and feel better if they 'work' to get their money.
Yes, when people choose a crypto-currency system to participate on a voluntary basis, it's not all about logic -- human emotions play an important role. A currency design may be very logical and efficient, but if can't win the hearts from people, it can't succeed.
Sadly, few care if the work they do is necessary or productive.
This is correct from utilitarian point of view, however, necessity and productivity may not be the most important factors. As long as the process is somewhat useful, engaging a large group of people is more important.
I tend to think that mining is the bridge connecting physical and virtual worlds: CPUs, video cards, electricity power, USD, hardware/software skills, hours and hours of frustration and enjoyment, eventually all of these get converted into a few virtual coins; this subliminal yet important bonding process is what Ripple network will always lack.