If the numbers were that low he would have done without. For me he needed them to be the tip of the spear due to how nasty things are and no superior firepower won't always work. It's my interpretation, but they were really needed.
I very much agree with you on this, and in relation to the OP, the Emperor was looking at the long picture for humanity in the galaxy. I believe he saw that regardless of incidental treaties and alliances, the most powerful species wins, therefore humanity either destroyed the other sentient species or they would eventually become their prey.
In one of the prior posts, someone said that the Eldar were largely ignoring Humanity by the start of the GC, since they were too wrapped up in their own thing, making the Emperor's emnity toward them less than understandable. The HH novels are full of references to Expedition Fleets saving newly discovered human worlds from Dark Eldar raiders who feed upon them for their sick amusement. I don't remember hearing anything about regular Eldar, but imagine they weren't any more considerate of humanity if it suited their purposes.
The White Zone is for loading and unloading only. If you have to load, or if you have to unload, go to the White Zone. You'll love it... it's a way of life.
: Cigarettes in space?
: It's the final frontier, Nick.
: But wouldn't they blow up in an all oxygen environment?
: Probably. But it's an easy fix. One line of dialogue. 'Thank God we invented the... you know, whatever device.'