All of my bike have triples and either 5,6,7 or 8 speed rears so I can't speak for the compacts with 10 or 11 out back. But....... I think the triple allows you to cover a wide range, have close spacing, and most importantly, not have to ride the entire cassette all of the time. With the triple you have more choices in how you can combine the front and rings/cogs. You can also stay in the middle of the cassette, ride 3 or 4 cogs and shift the front as you need it. Or you can do as you would on a compact and ride up and down the cassette and not shift the front much. You also get to keep the standard double with 2 bigger than compact rings and only use the granny as a bailout. I was doing a tri 2 or 3 weeks ago. I was on a bike with a 52/42/30 triple and 12-28 7 speed. About 90% of the bike leg I was on the 52 and the 3 smallest cogs. Occasionally I would go to the middle and the same 3 rings and maybe a 4th. Then right before the 1/2 way turnaround there was a short steep hill about 500 yards long. The hill was steep enough that there was a lot of people walking their bikes up. I went to the 30 and the big cogs and spun right up it going past plenty of people. Then I turned around and went back to the 52 and the 12 cog. The rest of the ride was on the 52 and the 3 smallest cogs again. Not much crosschaining and just some shifts up or down on or 2 cogs at a time. Had that hill not been there I would have had a cassette on in the 12/13-23 range for some really close spacing. As for weight penalty, you do have a bigger BB and a third ring, but that is offset some by the bigger cogs with the compact. Now, I have heard that above 8 speeds triples don't shift very well, so that might be a reason for the compact.

Here is a something for another thread. When I was spinning up that hill past those walkers I wasn't going much faster than they were walking. When I got to the top, I had to take a breather for a few pedals while they were refreshed after their walk. I know pride is a big motivator, but walking that hill might not have been a bad strategy. They did not lose much time and were refreshed at the top of the hill.