Well it is an objective measurement that is universally used by bullet makers and ballistic programs to give a floor for shooter and categories of game you can use for a given bullet. If they say on my new RWS Panther .177 air gun that it can kill to 525 yards then certainly a 50 cal ML bullet hitting an animal with 400 fpe, which is 1/2 the common floor value of 800 fpe is gonna kill a deer, they are really not that hard to kill in general. As bullets have gone faster and faster over the years, I have noted the jump from 800 fpe to 1000 fpe by writers and bullet makers, so there is a lot of truth in your statement, the extra 200 fpe does nothing to kill better. The chase for speed and distance has come primarily from the CF rifle community and betterbullets from that community. However on the lower end the difference between 200 fpe and 400 fpe is significant for penetration in my opinion and that is what is need to kill a deer---12" is the minimum for quick humane kills. Now i know 3" into the vitals in the heart area will of course kill, so don't think this is gospel. What is meant by 12" is that if you get 12" of penetration with expansion, that there is a lot higher chance of a quick kill. As hunters we want quick human kills, this maximizes recovery, we also want low meat loss, both of those things are working against each other in bulletdesign and fpe is sorta one way to get some idea of force delivered at target.

Energy = Weight times Velocity Squared divided by 450395 (fixed constant).





The Taylor Index is another attempt to measure this, because some believe fpe is biased toward "small fast bullets", because velocity is >>>squared<<< in the fpe formula that is true, it is NOT squared in the Taylor index.

TI = bw/7000*v*DI (2)
Where:
TI = Taylor Index
bw = Bullet weight, grs
v = bullet velocity, f.p.s.
DI = Bullet diameter, inches

This index is claimed to be biased toward "large diameter bullets" because of the DI in the formula:

http://www.loadammo.com/Topics/July02.htm

Bullet makers have to give "categories of game" that the bullet will work on. Anyway, I agree with you if your a "knowledgable hunter", but what about a new person just coming into hunting and has "no clue", we were all like that in MLing, at least I was.
Best Wishes,
Chap