usefull/useless related to the builds:
pvp
pve
single target
aoe
healer
dps
tank
aoe pvp
single pvp
also there is lot of different skills with different level of usefullness for different circumstances
you requested a book of 50 pages, its too much to explain
the most useless skill for any hero i think its passive points to
Keen Eye:Rune Stones