Peter (if he and jesus actually existed, as such) was just about as important to the creation of christianity as was jesus. He was, as you say, the loyal follower who'd stick around for anything. Without him jumping up to affirm his loyalty, the other disciples would've had time to think and might've figured out that they were following a new cult leader. Humans, as the social apes that we are, instinctively rely on the valuation of others when we decide on something's value. If a friend is hot after a girl, suddenly she looks better to you too. The same thing was probably in play here, the other disciples may have had some doubts about this new guy coming along and basically throwing their entire religion in the crapper, but Peter was always there affirming his loyalty and indirectly pressuring the rest of them to go along.
Personally this scripture always bugged me. Not just because the way it was used was always sort of said to me "This religion may suck, but everything else sucks a little worse" which was never satisfying, but more because of 2 Timothy 4:3 - "For there will be a period of time when they will not put up with the wholesome teaching, but according to their own desires, they will surround themselves with teachers to have their ears tickled." Whenever they read this scripture where peter's stated reason for sticking by jesus wasn't because he was the messiah, nor was it because he was backed by god as evidenced by his miraculous works, nor was it because he spoke "truth" nor was it because his "load was light" - it was because he had "sayings of everlasting life." How is that anything other than following someone to have your "ears tickled?" So it's OK to do it as long as you follow the right guy?