I don't KNOW, but I am willing to make some guesses.
eVent may be 'breathable', but ordinary Pertex or similar uncoated fabrics are at least 10x more breathable, or maybe 100x when you count actual air flow.
A major hazard for down bags and quilts is getting condensation inside the shell under very cold (ie winter) conditions.
A major method of getting rid of condensation from inside a shell is actual air flow: it carries the water vapour out with it. This is hugely faster than water molecule diffusion.
Many of us do not like having any coated fabric on the outside of down gear, for all the above reasons. In other words, yes, any air-breathable fabric is better than any coated fabric for the outer surface of down gear.