I pretty much agree with The Jackal, it all comes down to Blizzard's itemization and the fact that it is only a couple skills that they decided were good enough to use.

Unless the skill gets a 6set boost and has 1-2 other legendary items that fit with that set, then the skill is not worth using. And since everything is tied back to sets and those sets "require" a specific skill or two, you're pretty much forced into a few builds if you want to do anything hard/that will progress you.

There are a few "variables" in some, like "I can use any resource generator" but in the end that skill is practically meaningless on its own, it is just there to charge up some other skill that does all of the actual damage.

They've just designed themselves into a corner. A skill has to have a boost in a bracer, and maybe one other slot, so long as it is a 7 piece set rather than a 6, or you're forcing a ring pick too, which then limits your choices there too. How many different rings are actually worth using? There are 47 set/legendary rings and CoE, F/R, ORotZ, and RoRG are pretty much the only ones that see any use outside of maybe 1 specific build.

It sort of takes all of the fun out of the loot system when you're not looking for something better, you're looking for 1-2 specific items and the variable is if it showed up with the correct attributes for the build (and those attributes almost never change either),