When you are 150 yards away, you can move the little dot around on your screen to where YOU THINK the flag is, but you certainly don't know that for sure ... especially on holes where the green surface is hidden behind a mound or bunker. Just like k-troop also said ... you are just guessing at that point. Whereas, with the laser you are phsyically seeing what you are getting a measurement to so there is no question.
EDIT: For most of us, 2 or 3 yards difference is no big deal cuz we ain't that precise anyway, but as you get better, you are going to want to know exactly how far things are. Also, when I watch the "On The Range" show on golf channel I see caddies with lasers all the time, but haven't yet noticed one with a GPS.
The last course I played that didn't have at least different colored flags for Front, Middle, Back had greens so small it really didn't need them. Aim for the middle of the green was good enough. Almost all courses I have played in the last few years had 3+ quadrants for greens, many have 6 different pin placements. No you don't know exactly where the pin is but it is close enough for me.