New Vegas - How useful is high stealth really?

It's not wasting points.

100 stealth will get you much closer than reasonable sniping distance. However, there are quite a few enclosed areas in which you'll have to get a lot closer than normal sniping distances just to be able to see your targets (inside vaults, for example), and higher stealth will definitely help you there.

Furthermore, the number of points a high-intelligence character will receive by hitting the level cap is enough to max out almost every skill. There are actually a lot more skill points available than there are skills worth maxing; maxing sneak is hardly a waste of points, compared to, say, maxing survival in a non-hardcore playthrough.