Soldiers of the core nationality -- Romans, Mongols, Persians, etc. -- often fought as fanatics, to the death. Not so the draftees from conquered nations/empires/tribes.
Additionally, the Legion is portrayed as VERY hostile towards the use of higher tech weapons (which makes them futuristic Luddites: http://en.wikipedia.org/wiki/Luddite). In terms of domestic production, that might be acceptable, but certainly NOT when facing opponents armed with Space Age weapons. The Aztecs disintegrated when facing off with the Spaniards armed with a few score matchlocks, a couple cannon, and a handful of horses. Legionaries armed with machetes, some slug-throwers, and a few laser rifles would literally melt under the attention of a squad of Brothers armed with Gatling lasers. Furthermore, the Legion soldiers would KNOW that outcome was inevitable. There's nothing soldiers hate more than the thought that their lives are being literally thrown away with no hope of success. The leaders' staunch refusal to use readily available superior weapons would definitely de-motivate the rank-and-file Legion grunts.
Lastly, Caesar MUST realize that his mighty empire is doomed to disintegrate as soon as he dies. He's obviously enough of a historian to recognize the similarity to the Macedonian Empire, and what became of it when Alexander died. Knowing that without him, there is NO unifying force to hold all those defeated tribes together, one wonders what made him think that the cost of all that death and destruction was a worthwhile investment. If he did believe it to be so, just _what_ was it that made it worthwhile?