If you decide to walk around in say, poland, then yeah it wouldn't do you much good. But if you are suddenly exposed to radiation, it'll instantly blacken. If you expect to be going into a zone, or to come up against radiation, you'd take a geiger counter.
The film strip badge is just nice to have on the off chance you come across an unhealthy bit of ionizing radiation, and here's the kicker, high amounts of radiation in short exposure is far less lethal than moderate ambient levels of radiation. The chances of you being exposed to either is rather slim, unless say you feel like transporting some uranium, or walk into a recent melt down zone, or decide to pick a fight with a neutron bomb.
Actual low levels of ambient radiation is usually from radioactive fallout, which can easily be mitigated by wearing protective clothing, like a rain slicker. This would be the case where you have a geiger counter to avoid hot zones.