There were voices that raised concerns that FE does not do what it should do. I did a test shooting the orange dummy in Cotsworld as a scout lvl 47 with 47+5 longbow with a 98%quality longbow,

513 arrows with no FE gave me 11.3% crits. and
512 arrows with FE2 gave me 19,4%.
0 fumble, 0 misses

I conclude FE as works as intended. If I would have used more arrows I guess I would come closer to 10% and 20% what the expected values would be. (500 is so low that 2 more crits and you are right on target numberwise...)

So in the end for 4 RP you get 10% more crit (=59% average base dmg) ~ 6% in total for PvE. As crits are capped in RvR (50%).

Good for leveling once I hit 50, I might consider spending the points in MoA1.