Ironically perhaps, the best reason not to do this is "It is extremely likely that, given sufficient time, we will either figure out a way to use the waste besides high-yield weapons or a cheaper way to dispose of the waste."

Earth is huge. At the rate reactors create waste, the amount of land consumed by storing it is staggeringly low (the higher risks are transportation, which unfortunately the rocket idea doesn't solve unless we build a dedicated rocket site next to each reactor). Low enough to justify the risk-over-time of securely sequestering it instead of throwing it away.