I wouldn't say worst in game. Given a monster or rift guardian with 9 billion health, in 5 minutes the rg would be at 1/3 max life, or ~3 billion hp. Given this information is correct. We can find until what time interval this effect is optimal with a bit of calculus. The equation of bks effect is (the monsters max health)^(.004x). In the current patch there is a bug that if you die on the monster (still testing the specifics) bk will proc for what feels like atleast 30 seconds or enough for you to respawn in grifts even at the max timer count down with the ring still proccing. I have heard that there are more limits to the ring, such as it can only dot for 2000% of wep dmg and it will only stack 20 times but I am unsure if that happens. I know that it does stop working while you are dead at around the 30 second mark.
However, if we are working with the base assumption that its .4% of the current hp and you have a follower in the range as well it can proc even more getting the monster down below 30% in the range of the barb passive of 40% more damage which could increase the grift clear rates a considerable amount , given the current bug/exploit. Such that the time your dead you can still be doing .4% dmg per second.
However, if it can only stack 20 times then it's only effective for 20 seconds during death and even further if it's only good for 2000% weapon dmg , given a 4k dps weapon it would only proc 80,000 per second which is quite negligible for grifts or even rift guardians. However, when I have seen the numbers, I'll check tomorrow for sure, it seems more than 80k per second.
In essence it could be worthless or it could be worthwhile, more testing will be needed to be determined.