what are compact cranks for?

More expensive conversion. In addition to the new crankset, you would need new shifters and prossibly a new front derailleur.

I've never done those races but I would agree with your assessment that people may opt for a compact crank for IMLP.  I probably would (well first the rear cassette) because I'm just not a strong biker.  The thing is, not everyone will...many people will still stay with what came with the bike because they are strong bikers.  Oh, for compacts, you'll lose some speed toward the top since you have opted for less teeth.

The problem with the triple is not only is it heavier but also you'll probably have to change out or at least make adjustments to some components like shifters and/or front derailer.  I'm not 100% sure because I didn't decide to go this route, I went with the rear cassette change to a 12-27.  Also, the triple, at least on my MTB, always gave me more problems.  Not sure why, but the derailer just always seemed to get screwed up on the triple.

My old bike had a triple and my new tri bike has the shimano compact crank w/12-27 cassette. I love the new configuration...much easier to shift and better to climb hills with. I hate my triple and would never recommend it to anyone...harder to maintain, drops chain easier and more finicky. Compact has the same gear range but a little less on the faster end...but I don't really pedal that much when I'm going over 30 ...not an issue with me.

I live in MA which is hilly and having a compact helps me climb hills better and not burn out my legs. Max helped me figure out gear inches on my current bike and then compared them to different combinations on a potential new bike. Key was that I needed one on the low/easy end when climbing steep hills, keeping my cadence in the 80's and still make it up. Knowing my limitations or concerns on my original bike had my coach help pick out the right set up for my new bike, and it works fabulous.

Last year my normal cadence was high 70's or low 80's now I like to ride in 95-97 range so having a compact crank allows me to spin up the hills. When I get to be a stronger cyclist, I'll change my cassette. 

Didn't you say that yesterday (two posts ago)?

If you were doing an extremely hilly course - say either Six Gaps (10,000' climb/multiple ~15% grades) or the Brasstown Bald Buster Century (14,000' climb/up to 20% grade) - could you do a compact crank and a 12/27?  Would the gearing then be anemic on the flats?

Generally speaking, is your rear deraillure still compatible when switching from an 11/23 to a 12/27? (Specifically speaking, will a 10-speed Ultegra make the jump?)

Lastly, what are some of the varieties of compact cranks to choose from?  Outside of weight weenie stuff, what are the variable to consider for each crank?

Courtesy www.sheldonbrown,com, speed in MPH at 90rpm:

Standard 53/39 with a 11-23:

Compact 34/50 with a 12-25 (12-27 wasn't available in 10-speed):

The "anemia" would come in the upper gear, with top speeds of 34 vs 29. Chances are you can't hold either speed very long on the flats. You may lose something on the downhills, however.

Okay, that covers the anemia - and, for the record, I cannot maintain 29 mph on a flat - how 'bout the compatability + choices of cranks?

Yes. You can change the cassettes with little or no adjustment necessary, provided you're changing out one 10 speed for another.

I say if you need to climb a bunch of hills, get a triple.   Everything is covered at the minor expense of a few extra grams. 

Going from a double to a triple is more than a "minor expense of a few extra grams." In addition to a new crankset you would need new shifters. LOT cheaper to just get a compact.

See?  Apparently Bear was up all night converting his ride to a triple.  That's an awful lot of effort....

And whether you're putting a bigger casette on, or a compact, make sure you adjust your chain length appropriately.  Here's an article that'll help you figure it out.... sheldonbrown.com/deakins/lowgears.html