Not much serious training takes place in typical trial lessons which are more a famil or air experience flight. Taking two people and having an airborne crew changes means more airtime for the two studes, cutting a load of taxy and transit time. As long as it is properly briefed there are no downsides at this stage.
A little later when the guys are actually learning startup, taxy, takeoff, transit, RT etc then it has training value. Far beyond that when these things are second nature, for example when training beyond the PPL, there might be more value to a sortie by cutting out unnecessary items and again doing airborne crew changes. Very common in military flying training on certain aircraft types.
Always worth giving the incoming crewmember a couple of minutes in the seat to get his bearings and up to speed with what's going on before he starts aviating.