I used to teach science and I had a really cool set of materials on mammals. We did a comparison of some common North American mammals including deer. We examined fur samples up close and what I remembered is that deer have a lot of guard hairs (not good for soft yarn).
Out of curiosity and to make sure I had remembered correctly, I did a little research for those of you who are into animal trivia and found the following info from a booklet posted at the University of Georgia's School of Forest Resources
It confirmed that in the summer deer have ALL guard hairs in their coats. In the winter they have a different fur composition that includes soft underfur, but it still sounds like there is a lot of guard hair.
I'm not a spinner, but there is the scientific angle for what it's worth!