It depends on how you want to play, really. The 5th piece bonus won't help you at all against shielded targets, but can help you get the job done when enemy shields are down, as well as being particularly useful against tanks. It is one of, if not the most, efficient sets in regards to raw damage potential as long as you are not over-penetrating or hitting a shield.

For example, a fellow mag sorc may only have about 11k spell resistance. If you build to penetrate more than that already without Spinner's than the 5th piece wouldn't be helping you at all against him, while you could have been using something else for sustain or something that boosted your spell damage to hit his shields harder. Conversely, against a very high resistance stam character this would help a lot to actually get your damage through.

Personally, I run Shacklebreaker + Lich mostly, because stamina and regen are so important in PvP. Yes it's harder to kill tanks, but I can easily burst down weaker targets and have the resources to escape situations where I am outnumbered or up against someone I can't drop.

do we have comparison like this for stamina ?