I think a lot depends on the blend.
If everything is preshrunk first, (I'm a prewasher) and you like the way the fabrics go together, I see no reason not to use them.
Some of the blends do pill and that might be one of the reasons not to use them.
The synthetics also burn differently than pure cotton.
Seriously, now that I did a bunch of burn tests on my Mom's fabrics, I will never, ever use a polyester batting in a child's quilt again.
Why? The synthetics burn hotter and smokier, and leave an ash/residue that clings and stays hot longer than cotton.