True...but you can do the same thing with water changes as well. Besides, it would be very much the same thing as topping a tank off due to evaporation. Of course, I'm not trying to say it is good to remove half the saltwater and replace it with freshwater but I've always been able to do this with the aid of a hydrometer since it is a cheap and easy tool that can show relatively accurate changes over many samples; assuming that it is being used correctly. Additionally, I also perform this sort of thing by removing a 64 ounce (the super sized McDonald's version to be exact) at a time and replace it with the same amount of FW, repeating that until I acheive my goal.
IME, this is also a good option because it does not require a delay in time between noticing the descrepancy and curing saltwater for hours...which I wholeheartedly support doing like others who have mentioned the same. Likewise, though dilligent topping off with freshwater and mixing new SW to match the tank's sg and temp, this is a pretty easy situation to avoid. Anyways, I do agree with you in that there may be risks involved with what I am suggesting, but I feel as though they are no larger than the water change option. And for the record, I'm not disagreeing with the water change method, only that I have found my suggestion to be easier.