Caesar has an army that is almost perfect at asymmetrical warfare and he instead decides to waste all of its potential in New Vegas and field his army through a defensive bottleneck (the Dam). Even if you think the Legion would win the Second Battle for Hoover Dam without the courier, they would do so at a much higher cost than necessary. Keep that in mind, even if you believe Caesar would win the Second Battle that does not mean attacking the Dam head on was the best way to go about taking the Mojave.
For the purposes of this discussion let us assume all of Caesar's primary military activities would be in Clark County and the portions of San Bernardino in the game. So no saying Caesar should of just forgot about the Mojave and launched a campaign into Central California through Central Nevada.
Caesar has shown he has a great ability to deceive. As of now every NCR camp west of the Hoover Dam will fall with just the small amount of Legion forces west of there, the Fiends destroying Camp McCarran, the BOS taking back Helios, and presumably the rest of the Ranger camps malsupplied due to Hanlon, The real question to ask though is why does Caesar even attack the Dam then? Why not destroy the western Dam bridge like they do in game, destroy the eastern one, launch the successful western river attacks and send a sizable force by way of the taken Camp Golf and take Vegas which has already been weakened by riots and the Omerta's uprising? All the while the Cottonwood forces, and Legion raiding parties in that area would attack any retreating NCR out of the region.
What I just said would be the death of the NCR in the region. The NCR at the Dam would starve to death and would be generally unable to retreat, save for Oliver and his top men. Even if the whole NCR in the Dam managed to retreat from the Dam to save themselves they would be harassed, if not routed, trying to leave Clark County whether they went north or south. Regardless they would be in no shape to take Vegas from the Legion and would not get any resupply or reinforcements.
This is why I say Caesar is an idiot. He could of taken the Mojave as soon as the Fort was built and Cottonwood was taken and yet he waste time allowing the NCR's best (Veteran Rangers, Heavy Troopers, First Recon) move into the Region and into position. He risk losing or at least waste time, resources and men on this campaign that did not need to be wasted. Considering his medical condition and how little time he has to transform his Legion into an Empire letting his pathetic dream of "crossing the Rubicon" like Julius Caesar did to take Vegas, instead of just taking Vegas shows why even for a military dictator Caesar is not a particularly good one.