Although there are more than 76.000 cabs cruising the streets of Beijing, it typically only takes a few rain drops and it gets almost impossible to hail one. This means big time for all the private/black taxi (heiche 黑车) operators out there and we generally advice you against going with them!
It is potentially dangerous as you can’t really be sure about where the driver will take you once you sit in the back and doors are locked. In most cases, though it is just extremely annoying, once you have arrived at the destination, you might have to start an argument with the driver about the pre-negotiated price. You might well end up paying an overprice.
Another issue is liability in case of an accident. Your insurance company could opt out of paying because you took an illegal service.
Instead, keep an eye on the weather forecast, bring an umbrella, take the subway or the bus or wait patiently for the next official taxi to come along.