I could not even start to explain why someone would go to the trouble of stealing something like this unless it’s one of the local 15 year old punks just causing trouble.
The downside to not having a cap in place is that it allows dirt to collect on the Schrader valve through which air is added. The next time an air hose is connected this may push dirt into the valve seat and the tire will lose air pressure due to the leak caused by the dirt.