What about homing? if you have multiple motors running off one output and you need to home and you have a switch on each axis, how does it handle that?
If you have a gantry and have a motor on each side connected to one output and have one home switch, how will it be able to home both sides accurately?
Also the more load you put on the output the more likely it is going to be susceptible to electrical noise and then you may have one axis loosing a step or two and over time this will add up to a big error.