I can't speak to the accuracy of the putt, that is still dependent on the read and how much break you'd play. But the consistency of length is the issue here. With a 15' scale, if you play it for 10' the ball will roll pretty close to the same distance +/- 6" almost every time. If you use the 30' scale, now it may be +/- 1' or more if you don't hit it exactly perfect. The ability to use the smaller scale is for more precise distance control, IMO. Hopefully that is what you were looking for.
Using the higher scale is fine for short putts because you can ram it into the back of the hole for the most part. But if you have a HUGE break and you miss it, watch out for the 8-10' comebacker!
Good luck and happy golfing
~ProJoe