In this formulation, I think it's a pretty reasonable comparison, since a prospective user doesn't have to sacrifice much (just a semi-luxury item, one day a month) in order to use your service. The author's criticisms don't seem to hold as much purchase (no pun intended) in this context.
I'd be curious to know if others think of cup-of-coffee pricing as $5/mo or $5/day. The only context where I can remember hearing per-day pricing is life insurance or supporting a child in a developing country.