As long as you have enough pins that long one device will do the job and you wont have to use mcp3208
I think the reason to use mcp3208 in general is not only to extend the number of axis but also to use 12bit adc instead of built in 10 bit adc.
As far as i am aware pro micro offers 10 bit internal adc (axis sensor) which offers resolution of 1024 and mcp3208 is a 12 bit adc with resolution of 4095.
