The good side is that with zsh and some other software getting their place in OS/X; Linux and OSX communities will diverge away from each other. I kind of like this as a Linux user and developer. With OS/X or Apple we share so little in principles (read a lot of comments here), vision and development(now) and it is good if we end up being 2 mutually exclusive ecosystem similar to Windows/Linux.
Being a former Apple employee and having enjoyed Apple's ditching of the classic Mac OSes for OS/X, both worlds have had pretty much similar environments. After 2012 the Apple and Linux development communities appear to be totally accelerating in different directions.
For Linux and OS/X, noone said they share common parent; the shared common tools such as bash, samba and a lot of stuff removed as mentioned by this article will create diverging ecosystems (and before replying watch out, noone says diverging kernels).
There are plenty of other shells out there as well that they could replace bash with. Bash isn't the end all be-all of shells.