(He he he... he... oh dear
Out of curiosity why would that be so terrible? I'm not all filled in on lore.)
Steve is the Gangster Computer God that was responsible for humanity's survival and eventual victory in the Altered Wars and helped make many of humanity's successes possible (or so he claims). He is supposed to be a very good administrator, capable of handling the governance of an interstellar empire's military and espionage operations far better than a human government could. He had control of much of UWM's infrastructure and was able to sabotage it, which is part of the reason we haven't lost the war yet. Plus, he has lived long enough and has access to enough information sources to have an incredible wealth of knowledge. He is our greatest asset in this war. If our enemies in the UWM were to get a similar system (which I will refer to as Stephanie) or if something were to happen to him then our greatest advantage would be rendered null.
Even worse, AIs can make a "battle of the minds" and sort of assimilate each other. And since almost our entire rebellion is directly connected to Steve, were part of him (or worse all of him) assimilated by her somehow, it would mean that our enemy would gain a great amount of knowledge about us and the ability to more easily disrupt our operations. On the other hand, if we were to gain knowledge of the existence of a hypothetical Stephanie and successfully manage to launch a covert mission against her, then perhaps Steve would be able to assimilate her and thus covertly gain control over our enemy's assets.
And assuming Steve is saying the truth (that he is still following his original directives) and this war is for the survival and betterment of mankind (Steve basically told us that humanity kinda failed after the Altered Wars and there was nothing much he could do about it except plan and prepare this rebellion through HMRC so he can wipe the slate clean by destroying the old government through war and creating a better one), then Stephanie wouldn't help humanity get better as much as Steve, because she'd probably be shut down as soon as she completed her objectives, out of fear that she'll end up like Steve.
Irrelevant, but even if we loose this war we'll be helping mankind, since we'll be stimulating the UWM to get off their asses, work together, reform their government, coordinate and actually do something for a change instead of resting on their laurels.