Some fields allow them, but it's one of those things that you use at your own risk.
I normally don't like mesh eyewear, but I've been testing some that Overwatch's "Black Market Supply" has gotten in at the field at Bad Karma and they work great at longer distances. While I was reffing I was hit from about 75ft (I consider this a close distance for field) right in the face, and the BB only left a small scuff on the mesh. (Here is the type I've tested:
http://shop.ehobbyasia.com/combat-gears ... black.html) I also shot them from about 150ft away with my sniper rifle (550 fps on .4g) and it only left a small dent which was easily tapped back out.
As stated in a previous post, I would never recommend wearing mesh goggles in a CQB environment, or even in field where you may enter CQB type areas.