If Cubase wants to stand against Ableton Live, it will have to incorporate some things so we can use it in live situations and being able to adapt the BPM in real time is a critical part of it.
I already discussed it as a feature request for Cubase 8:
A use case would be: the band is playing something, you sequenced something in cubase and now you want to play it live. Instead of forcing the band to match your click, use MIDI to tap tempo to set the bpm and then click play.