Drafts in modern day are generally most helpful for defense and keeping the homefront strong so that the main military can focus on the offensive without worry of HQ being unprotected. Most modern wars are admittedly also much smaller-scale than something such as a World War, so if WW3 were to happen the scale would call for a larger bulk of soldiers...partially because the red-tape that slows down modern war would be cut away due to survival necessity. War is odd and heavily condition based.

Also the draft is a bit of a complex issue, making it based on able-bodied adults would be more fair, however it would make it much more complicated and frustrating than it already is with the current "If you are 18 and male, sign up.", it would probably need some odd official test and an official looking over a bunch of paperwork you have to work through to figure out if you are part of the draft or not...its a bit much for someone who just turned 18 compared to 15 minutes on a government website. Not only that but while the military is a bit more professional, a militia including both sexes could possibly be chaos, and generally women are admittedly far more complex in compensating issues. Pregnancies, periods, more likely raising children than men, less consistent physical abilities (While some women are strong, the fragile ones are so much more fragile than weak men), bathrooms, in current society men are more conditioned to "suck it up" and deal with their issues, not to mention the drama of these less-professional soldiers interacting with eachother...I'd be scared to find out how much rape actually would happen...In the long run it just seems best to avoid the issues of trying to make men and women interact in this kind of situation...I feel like I am only touching the tip of the iceberg here, too.

Plus who knows how they would incorporate this system into society, would the millions of women in the US suddenly need to sign up? Or would they have this really confusing and unfair feeling thing where only newly-18 women have to sign up?