His advice is....ridiculous. Really you only need a double if you are in flat lands. A triple is nice in the mountains, but you may be looked down upon, until you spin up the mountain. The triple actually gives you bigger gears 52x39(or 42)x30 than a compact 50x34. Triples are also heavier. A compact takes a little bit of each.
Generally, *Generally*, your body is more efficient at higher cadences in the 85-95 rpm area. I weigh 180 lbs and can out climb a lot of people 20-30 lbs lighter than me on my team WHEN I have my triple (unfortunately, that bike is temporarily out of commission). Why? Because if my cadence drops below 75 I really start to hurt. I easily spin at 95 and can go long periods at 120+.
The negative side about a compact is on the high end. You only have a 50 tooth chainring rather than a 52 or 53. That can take away top end speed on a decent (pretty much the only place--few people at our ability can efficiently sprint on the 52x11), but really if you aren't racing, that doesn't matter.
Triples are also more finicky on shifting and shifting adjustment.
I can't even understand his rational. If he really knew what he was talking about, he would suggest getting a new cassette (heck of a lot cheaper--the dent in your wallet is a lot deeper to switch to a triple) is the way to go--something with a 28 tooth (just make sure your rear derailleur can handle it).
Define serious cyclist? Pros? Rarely do they have a triple. Amateur racers? Same thing. Look at most high end bikes at your lbs--few triples.
My advice? Ride more, pedal harder, leave him behind at the next sprint and tell him he may need a triple.
To clear things up, I still do love my triple. Nothing wrong with it.
Ignore your friend's "advice".