Teams should always strive for L-R pairings in the modern NHL, and that's what they do.
But, the reality is that there are more LH NHL-caliber defensemen in the world than there are RH.
And, yes, there are a few lefties who prefer to play the right side. So yes, it's often that teams have only 2 righties, because ultimately you're right, you're not going to dress a shitty righty over a serviceable lefty, if the only penalty is having the lefty play his off-side.
Right now, the Wings have 5 worthwhile NHL defensemen on the roster. Three lefties and two righties. If they were trying to upgrade on the trio of misfit toys currently occupying the 6th spot, they should
try to add a righty. But if a lefty were the best option, so be it.
Also, last time we won the Cup we absolutely had three righties. Rafalski, Cheli, and..... Garret Stafford played 2 games