From an environmental perspective, these kinds of economic incentives seem highly damaging. Not that I've got anything against you for exploiting them.
The planes are flying regardless of the people seeking frequent flyer miles. Aside from the marginal fuel increase due to the increased load, there's not much impact.
the vast majority of harmful vehicle emissions come from freight airplanes and ships with commuters coming in as a distant second, so even still the impact of consumer air travel is a rounding error (if that)
There's also the signal to the market that this flight is in demand. If you're only on the flight for points, (not the utility of the actual flight) then you're making a purchase without it actually being in demand. How this plays out - the airline seeing demand and adding another plane, or having more passengers so they can drop their rate to cover marginal costs and encourage even more passengers... I don't know. No judgement, just saying there's more to it than the jet fuel.
Plus on the redemption side, award tickets are almost exclusively "unsold inventory" -- a strict market signal that if a lot of people are redeeming and not paying, that flight should be either switched to a smaller aircraft of scrapped entirely. From that perspective, flying almost exclusively on frequent flier miles might be the most environmentally friendly way to fly ;)
