so why do some pieces have set bonus while others don't?

Cause different gear since there is four set none set for 5th spot is probably better then tier gear .. Is my guess

Kind of like how shamans used the other spec gloves for pvp like they did for most of the seasons. Obviously the stats are more beneficial on the off piece they are using instead of the set piece. So you guessed right.

I wish pvm sets had different bonuses, like you could choose a set that has a certain bonus based on what YOU think is best. Would add more personal preference to gearing and how your character works.

most people won't agree, but I like that idea.

and I haven't pvpd since early cata so I'm sorry for those who do:(

There is one set with a heap of off pieces that dont add to the set bonuses, used for min/max'ing.