There's a long running debate on these forums as to how necessary cycling clothes really are. It seems to boil down to how far you ride, how hard you ride, weather conditions, personal anatomy, and personal preference.

The longer and more challenging the commute, the more likely you'll find some benefit from cycling shorts. On the other hand, there are people who do fine without them no matter how long the ride.

Personally I use them. I have access to showers at work and keep them in a gym bag in my office until it's time to go home. They get sweaty and while I suppose I could just pull a pair of pants over them, I would feel pretty grungy. They wouldn't dry well that way and walking around all day in damp shorts isn't probably the best thing for you skin.