To be 100% honest, I don't believe I can give a full answer to that question, but I will give it my best effort. I was an electrician in the Reserves. I barely had one year of active duty in 8 years combined. I can say that we were trained in defensive tactics, given that we were going to be stationed in supporting positions no where near the front line. If I were involved in urban warfare as is so prevelant now, I would definitely be interested in having a pistol because of the cramped quarters. But that's really the job of Special Forces and the Marines. The Army, in general, does not engage the enemy in those surroundings, so a pistol would be pretty worthless.
I will point out one other benefit to the rifle that may not be too well known. The M-16 round is considered to be a non-lethal round, in the short term. It's a small-calibre, low-velocity round when compared to traditional assault rifles. The result is that the round enters the body and bounces around instead of exiting. The internal retention of the round causes damage that will most likely result in death, but in the short term, it simply results in an incapacitated soldier.
The reason this weapon is designed this way is two-fold:
- If the wounded enemy is incapacitated, it takes at least one other person to care for that soldier. In effect, you've doubled your "kill" total.
- If the soldier is left to die, he will live long enough to face interrogation when captured alive at a later time.
I mention this because pistols are traditionally used as lethal weapons (again, speaking in the short term). A pistol-carrying soldier is strictly less efficient than a rifle-carrying soldier using the criteria laid out above.
As for the cost, I spent 6 of the 8 weeks I was in Basic Training on being trained with my M-16. This involves more than simply shooting the weapon. You have to learn how to care for the weapon and how to use it in combat situations. The cost to upgrade all soldiers to both rifle and pistol users would be extraordinary in terms of time and training alone.