I used to think so, too.

Since I have been cycling I have strictly relied on carbs while on the bike. Recently, I have added small bit of protein to my "long ride utility belt" and I "feel" much better and finish stronger. Never heaved; nevertheless, the kind of protein you use may cause digestion problems, but once I sorted that out, I became convinced based upon on my own trials. Try it out, can't hurt. Some say 7:1, others 9:1...

not a plug, 'cause I don't use it, but the reading is interesting. Click.