I saw a promo for one of the "earn miles if you buy a room from us" sites not long ago.
I have a trip planned so searched on both that site and my usual travel agent.
The cash price via the points site was more than the OTA price. When, out of interest, I checked the cost to buy that number of miles from the airline the cost of the miles was very close to the extra cash price. So the miles aren't free at all they just enable you to get around any buy miles restrictions the airline programmes have.
Paying over the odds with your own cash is fine but I wouldn't do it if my employer was paying. They might not be very forgiving.