What makes you think that a military victory is what the Dark One wants or needs? Most of the forsaken think its important but they don't get with the DO's game plan is, Moridin confirmed this. Most of the forsaken and darkfriends think that the DO will use the shadowspawn to rule the world. He wants to destroy it.

We have already seen a portal stone world where the Shadow won the Trollock wars but the DO was still imprisoned. Personally I think that the freeing of the DO will have more to gettintg Rand's the lights appointed champion to do something at SG at a certain time. Everything else from the bubbles of evil, and the famine and vast armies of the Shadow are just about distracting the forces of the light from the real battle and keeping the darkfriends on side thinking that there will be a world left to rule.

If the DO did not have a big army some of his followers might start to get suspicious and if they realised what he intends evil or not they will probably abandone ship for self preservation if nothing else.