Being able to range either by making an educated guess or milling the target is great, but the accuracy required makes an LRF a very good thing to have.

The following is from Zak Smith of Demigod LLC:

Knowing the range of the target is critical to making long-range hits. Between 500 and 1000 yards, the best .308 Winchester loads are dropping at one inch every four yards forward travel, up to one inch every yard forward travel. For example, at 700 yards, it drops at about 1/2" per yard. If a target is 16" in diameter (a pretty large target) and the shooter aims at the center, he has 8" of vertical error margin.

This margin is used up by rifle and shooter accuracy, accuracy of "data book" drop values compared to current real conditions and load, and ranging error. If we look only at ranging error, that 8" of error margin translates into 16 yards. In other words, the estimated range which is used to look up the drop value, must be within plus or minus 16 yards of the real value. If his range estimate is 25 yards short, the bullet will be 12.5" low, and it will miss the target.
Obviously he is discussing small arms and not crew serveds/rockets/grenades, but the principles of trajectory stand.