Its usually a good option only if you're topping up points to redeem them. For example if I needed 15,000 points to get a hotel room that is regularly $120 and I had 14,000 points I would spend $10 (penny a point). Its usually not a good value, its making the best of a situation.
Since points generally have expiration dates you can't always "afford" to wait till points accumulate if you're a low accumulator. That's when buying points becomes an imperative.