I don't hate Nash, but I don't see him bringing a lot of value either. He's a low event player. Doesn't generate any offense, doesn't make many mistakes. Good at the dot and good on the PK. I agree with BD that he's an offense-killer. Even Marchand and Pastrnak couldn't score with him at center.
I think if you have a dynamic top9 like the Penguins or Lightning he'd be a very good fit on their 4th lines. Those teams have impact players on their 3rd lines (Kessel/Johnson), so they don't need goals or even momentum changing shifts from their 4th line. Just be a placeholder for 8-10 minutes.
I don't really know what the Bruins are trying to accomplish with their forward lines but they don't have the depth (of impact players) to waste those 4th line minutes. They need to get something from their 4th line, whether it's a little offense or a jolt of energy - and Nash brings neither.
At this point, I'd rather have Schaller as the 4c. He's not as strong at the dot, but he is just as effective on the PK, he creates a little more offense and he's be better at playing with skill if they need to move one of them up for an injury replacement.