I spend a lot of time in the shell and it's so incredibly rare for what I'm doing to be limited by bash that it's hard to describe switching to zsh as anything other than a premature optimization.

Part of this is because I try to automate anything I do more than once or twice and I use a real language to do so. For anything non-trivial the differences between zsh and bash are an order of magnitude less significant than either compared with Python, Ruby, etc.