Depends on how effective you are at connecting with heavy attacks in PVP. As most are easily dodged, I would go with not very. Although, I suppose you could make a nightblade who would sneak up and gank with a heavy attack....

Here's some more info on it:

https://forums.elderscrollsonline.com/en/discussion/410844/heavy-attack-bonus-for-sergeants-mail-set

I would say there are better alternatives