There are a number of reasons why women should not serve. First of all is the immodesty involved when men and women serve together in such a capacity. Women are also less strong than men when it comes to fighting. Also, it is dangerous for men to serve along side women in such cases because they will be distracted from fighting in a life and death situation. They wouldn't be able to concentrate on fighting. This is the same reason why men and women should not pray in a mixed minyan. If women want to serve, it should be in women only units with women as their commanders and not be subordinate to men who will take advantage of them sexually and otherwise. But of course women only units should not serve in combat duty.