If you are are in the environmental biz, how important is it for you to wear "branded" field clothing?
This has varied considerably in my experience. I've worked at places where all of your PPE (personal protective equipment) had to have the correct logo, but nothing else mattered; places where essentially all of your clothing and PPE had to have the logo; and places where not only did management not shell out for branded stuff, but if you somehow acquired and wore something with the organization's logo, you were looked down on as sort of a management shill.
Most of my skilled labor contractors (excavation/construction, drillers, waste disposal) wear all company-branded stuff all the time: hard hat, safety vest, shirts (from button-downs to summer t-shirts to sweatshirts) and jackets. Most of the consultants I know will have the logo on at least safety vests, because they're generally cheap, interchangeable, and easy to put a logo on. Many of the specialty firms I've worked with don't seem to have any branded stuff.
Using company-branded stuff is convenient if it means that you don't have to buy/maintain stuff that you wouldn't otherwise wear. But the problem is when you're not a standard size, or you need stuff with better performance than whatever is being handed out. For example, I'll happily wear the corporate t-shirts. But when you're a lady with short arms and you need some more technical gear for cold/rain/strenuous activity, getting corporate stuff that actually fits is a bit of a crapshoot. When I have worked for institutions that required logos for all field gear, I did what I could. But once the weather became beastly, all bets were off. I wasn't going to get hypothermia over a logo.