Now minimal speed set of Geoscape is 'very-very-slow'. But not 'stop'.
I understand it is slightly out of classical style, but to me Pause Button is very useful in 2 situations:
a) Out-of-game event. Stoping in-game time to answer a phone or, possible, make a tea/coffee etc. Now pause for such situation can be emulated by entering Load/Save menu but I don't like it - Load/Save menu is for Load/Save.
b) In-game. If Geoscape pause is a 'natural' mode, it will eliminate so hard-to-fix problem with simultaneous air combats, popups from base event when user is in base view, etc.