The idea that a mandatory draft promotes equality appeals to a traditional concern for equal treatment— However, it romanticizes the extent to which conscription is and has been an equitable practice. For example, simply looking to the American Revolutionary War indicates that although select states drafted soldiers, they let well-born conscripts hire replacements. These replacements were most often those who were poor and jobless.
Later, the American Civil War also saw an implementation of a draft by the Union. Yet, even here conscription was hardly fair. Instead, draftees could escape service if they could pay a $300 notice, a large sum for individuals in the 1860s. Apprehensiveness regarding the draft continued in both World War I and World War II until finally reaching its peak during the Vietnam War (1969).
It was here that the "jerry-built system of deferments" forced the lower classes to face combat disproportionately. 
Although draft reform eventually did occur (and was altogether dismantled), military conscription has historically been unjust. It simply operates under the illusion of equality.