It's always a good idea to carry something to inflate the spare tube. Also, I carry a patch kit in addition to the spare tube, on a long ride it isn't particularly uncommon to have more than one flat. The guy I rode with yesterday was having a bad day, ended up with three flats.

There is definitely advantages to wearing proper cycling clothes. Cycling bibs or shorts provide padding as well as being aero. On top of that they help avoid saddle sores. The jersey is a big advantage as well. The pockets in the bag come in very handy, the jersey wicks sweat, and you can unzip it on particularly hot days or just on a climb when you could use some extra air.