Have you taken a close look at Rolling Grenades vs. Widow's Kiss? BW was the first character I ever played, and I really enjoyed it because I really didn't spend much time looking at the stats, but instead just did what was fun. I was making liberal use of Rolling Grenades because honestly it was just great blowing everything up.
I prestiged to try out the new build and now I am looking a lot closer at the numbers. There isn't a new calculator out so I can't see how everything scales, however, so far I am seeing the following:
-Rolling grenades is now a dot where it used to give all damage at once
-Initial damage is pretty similar between RG and WK
-dot damage is better for WK and it lasts two additional seconds
Plus you get the weakness for WK. The only thing I am not clear on is how damage stacks for RG. If a mob is caught in the explosion of 2 or more of the 5 grenades does the initial damage and the dot stack for each grenade?
At this point I'm not clear on any advantage to Rolling Grenades and if it is even worth a spot on the bar anymore, with the exception that spamming Widow's Kiss is pretty boring

The only big difference is the way the grenades roll out, and they can be good for just dropping them into a group and then moving the other way to create some distance.