As someone who uses it for medical purposes:
Without it I have a hard time functioning let alone doing my job. The caveat here is that I use only as much as I need to function. I usually take it at night a couple hours before bed. By morning the high is gone, I can drive, have good control over my medical issues and I can work during the day. Compared to regular medications I get from a pharmacy, I have many less side effects and can deal with them easily. IF USED RESPONSIBLY, I do not think it is any worse than drinking and like anything else if abused it will cause problems. Coming to work, driving, etc while high is just as unacceptable as doing the same while drunk. I agree, toking up before driving is not acceptable at all. However, I do not see the issue with it being legalized. Granted they are still doing research, my personal experience is that I can tolerate being a little high much better than I can drunk. No, that does not excuse someone coming to work high.
The government and companies are so worried about things their grandpa said about pot 50 years ago that wrapping their heads around why people use it seems foreign to them. IMO that is very unfortunate, especially for people like me who use it to function. It limits where I can live and where I can work. Sucks.