it all depends on what you use your portable for, is it a cleaning machine or a rinsing machine.
if you want to use your portable as a cleaning machine, so the solution; dissolves, detaches, encapsulate the dirt in the carpet then removes it, you will need heat.
if you have already 'dissolved, detached, encapsulated' the dirt from the carpet before you get the portable out of the van by using an appropriate pre-spray & agitation machine then the portable is used only to rinse the dirt out, so heat isn't necessary.
this is why truckmounters don't like M/S they see their machine as a carpet 'cleaner' not 'rinser' so don't want to be wasting time agitating.
given the choice i would chose extra vac power over a built in heater.
Mike