The main reason for not operating with dissimilar goggles is so that both pilots or pilot and nav can see the same things. If one guy is navigating and trying to talk the other one on to the LS, it is fairly entertaining when the handling pilot can't see what he is being told to look for. In a high threat environment, all the crew need the best visual acuity they can get and having one person miss an enemy position/tank etc because he had the crap goggles could lose the aircraft. So for military ops, generally using mixed goggles is a false economy.
Also the safety aspect of flying with one set of goggles one night and another the next just means more potential for cock up in the cockpit when the battery fails and you can't instinctively go for the battery switch because it's in a different place each time.
For HEMS/police work I can see no real safety case for avoiding mixed goggles, it's just nice if you can, that's all.