If I had a looking glass into the future I could predict what I should and shouldn't do. The integrity of any given decision is relative to your current situation. The background info on the NCR's intel of neighboring factions is thin so nobody really knows whether they knew of the approaching horde of Dick's sporting goods patrons. The NCR's position at the time was that they had to root out a heavily armed & armored group of soldiers sitting in their backyard. They wanted the power plant so nuking the crap out of them was out of the question. Assuming they knew the Legion was on their way: their decisions become more grim because now they know for sure they'll be caught between a rock and a hard place. Football Jockey's on one side and King Arthur's discount knights of the round-table on the other. In the context of having the big guns and technology, yeah one can argue that probably throwing precious manpower into funnels is a poor strategy, but the NCR essentially would have lost the greater war against the brotherhood had it not been for manpower. It was the only viable option.