IL - Why do you want to limit mines to one per area? AA guns are not limited like this. I was thinking about Russia using this as Germany approaches her cities, but I would rather use the $6 to buy infantry who can take a hit and can also fire back repeatedly. At the most I would spend $6 on taking out 3 German infantry. If I can place more mines and possibly get a chance at hitting tanks or artillery then it is more appealing to me, and I am forced to choose between that risk (of not hitting any) and placing more Russian infantry.
In the sea it makes more sense because any ship you hit is an expensive unit comparable to one mine.