As much as i remember is spinners the best damage set for magblades in pvp. The reason behind using spinners, warmaiden or silk of the sun over axiom is, that you need all those sets only on one bar, while its better to keep the last boni of axiom always active. Since most magblades use a destro and resto, a set which only needs to be frontbarred can make 5/5/2 setups possible: an example would be 5 spinners on frontbar and 5 lich backbar, so there is still room for a full monsterset like skoria. The same goes for warmaiden. Both sets mostly boost your damage, but not your healing, so the last bonus isnt used on the healing staff. Therefore you can complete another 5 piece set on the backbar like lich or wizards riposte. It is similar to the magdk and silk of the sun: if you slot all your fire abilities on one bar, you enhance them all by wearing silk of the sun only on that bar. But the last bonus isnt needed on the other bar, if there are no fire abilities. So once again this makes up room for different setup styles like sword and shield plus resto, while you can have one whole set always active and sun only on the damage bar. In this case sun is better than axiom, since the other boni are better. Same goes for spinners and warmaiden. In the end axiom is very good, if you keep it active all the time, because it may also buff your healings like dragon blood as a magdk. But a magnb heals with healing ward mostly, which isnt affected by axiom. Those are the reason why other sets are used more than axiom.
Ah btw another set to only have backbarred would also be the craftable set eyes of mara, which reduces the cost of resto staves abilities (also the ultimate).