I think that one must train as much as possible symetrically when sparring. We all naturally tend to emphesise one side. if our goal is learning how to fight in shortest space of time, then 1 side only is logical. If our goal of sparring is to reach the level of spontaneous action, without reaction without conditioning, then symetrical is key. You cannot do this properly if your spirit/body tells you systematically "oh no! he's attacking me on the wrong side, I must pivot and turn to attack him with my heavy right hand"

On a very practical and concrete level, when sparing, if you have the habit of grabbing with left hand and striking with right, (as I have seen some schools train exclusivel this) you WILL get nailed all things being equal by opponent if he knows you can only throw a punch on one side especially if you consider leg strikes.

Furthermore, there are clear health problems associated with one sided training in my experience. This is why we are taught, even in one sided forms as are present in basic Pei Mei, to practice opposite side version at least once if you practice one side version many times in succession.