Regarding risk when down to wear indicators, they're there because when "serious" riders finally started riding clinchers, they rode them a lot. The first indication of any problems was with mountain bikes - with mud etc the rims would wear pretty quickly. They'd ride them until the rim sidewalls were carved out by the brake pads, sort of like a worn disc brake on a car. Then, one day, the rim sidewall would spontaneously let go, and there'd be a big explosion and sudden lack of control.

Wear indicators were first extensively used on mountain bike rims. Now they're on road rims. They are used to indicate when the manufacturer will disavow all responsibility for any problems arising from continued use of the rim. Or something like that

Personally, if the rim looked/felt thin (i.e. without a tire on, or just gut instinct) I'd get a new rim. I've seen the aforementioned blown out rims and they are not fun at all. A foot of the top bit of the rim (the hook area) like a big "C", separated from a thin worn out unusable rest-of-the-rim. Usually a big blowout, some aluminum shards, and a big "wtf" before figuring out how to get home.

cdr