They already made the jump to 120Hz on iPad right? I'd love to make the jump to Macintosh computing but the refresh rate is a deal breaker. Due to my Windows set-up I am used to 144Hz refresh rates and hence everything else looks very cheap and stuttery to me.
The issue is bandwidth. The 5K iMac uses 10-bit color at a large enough resolution that you need Displayport 1.4 to carry the signal, or two DisplayPort 1.2 channels, if you are using 60Hz.
To do 120Hz on a 5K iMac means dual DisplayPort 1.4 channels. Oof. At that point, it would be impossible to put the GPU to sleep and use the integrated GPU to drive the display, consuming more power (Intel currently still only supports 1.2 on their GPU blocks).
The 15" MacBook Pro
should be able to do 120Hz, but it'd need dual 1.2 channels to do it (similar to the 5K iMac's screen). It could use DisplayPort 1.4, but has the same issue where you wouldn't be able to sleep the dGPU, which would be an even bigger hurdle on the MacBook line than the iMac, and noticeably impact battery life.
It looks like you'd also probably drop from support for two external 5K displays to one, as a result.