First of all, I do not want to exclude other ways of spending the auction revenue, but only to use part of the funds for refund to applicants.
However, for those applicants who applied for new gTLDs for REAL usages, ICANN clearly over-charged them. This is not ICANN's fault, but ICANN's original calculation of costs was too high without considering the auction revenue which happened beyond expectation. Thus, we OWE them a refund.
Therefore, as ICANN's original purpose of the new gTLD program was to facilitate usage of new domain names, refunding those who paid the hefty $175K for real usage of new gTLDs would only be natural. In addition, if ICANN's refund is proportional to the real usage of domain names but excluds those being parked, it could motivate registries/registrars to discourage domain parking in the future.
Furthermore, it is already recognized that the new gTLD program also has its down-sides.
One of those is trademark holders are often forced to spend money to "defensively register" domain names in new gTLDs, with some of the costs substantial. Thus, using some of the auction revenue to subsidize those who suffered from this new gTLD program would also be reasonable.