there are several pros with 650C wheel on a smaller frame. frame geometry doesn't have to be compromised to get the wheel to fit. biggest problem I've seen is very short head tube length. Shorter it is, the more wear and tear on the head set not to mention mor finicky in adjustment.

As tri-girl says, 650C wheel definitly has advantages over 700C wheel in accleration. You can "spool up" much faster on 650C. It takes lot more energy to spin up the 700c then it is with 650C (its physics of things and unless you are god there ain't much you can do about that). Faster spin up means faster acceleration. The whole thing revolves around a thing called angular momentum.

My old Ti bike had a 650C wheel and it was comfortable.

cons: choices of wheels are limited and less manufacturers are building 650C wheels. less choice in tires too. I wanted velomax orion?/circuit? or asscent wheels but they only came in 700C (I think velomax is part of easton now). I just checked yes velomax is now part of easton hmmm they now have 650C asscent/orion/circuit.

If you race see eden's comment. also at UCI sanctioned race, 650C wheels are not allowed for TT's (atleast that is what I remember).

Go with what is comfortable. They are still enough choices for 650C wheels.
And I think Tri people still prefer the 650C wheels so it wont be discontinued any time soon.

smilingcat