I'm mostly with maxwell on this, but I put it this way; IF you can find a cheap pair with excellent optical quality, good fit, scratch resistance and that certain je ne sais quois, buy two pairs.
Flyfishing and fly tying are things that require small acts to be done with great concentration. If you're losing your glasses, sitting on them, tossing them under your fishing buddy's foot, then you've got a problem besides glasses. You're also gonna be the guy or girl who drives off with her flyrod leaning up against the truck. Don't ask me how I know this, I just do.
-flyangler, who hasn't lost a pair of sunglasses since the year she propped her rod up against her truck and drove off.