What kind of question is this? (And :shocked: at the responses)
Maybe the question being asked should be "Why ISN'T a light pedal ideal?"
I can think of a couple reasons why it IS ideal:
1. Lighter clutch pedal effort usually implies less stress on clutch master cylinders, clutch slave cylinders, lines, release bearings, and crankshafts.
2. Lighter clutch pedal effort will usually promote greater driver focus. (eg. 20 minutes into a session, fatigue of your left leg is going to be a distraction. Regardless if conciously, or subconciously, it is still a distraction. The same can be said in stop/go traffic.)
3. The time old argument of "overkill". Which would you rather pay to replace: Transmission gears/synchros, or a clutch? "Just enough" clutch (regarding torque witholding) is, well, just enough (unless more agressive friction materials are needed to withstand the stresses/heat of the given application.)
Bullet #2 is also directly related to driver comfort. An uncomfortable driver will make mistakes, and be slow around the track. (The series of "Speed Secrets" books by Ross Bentley explains this in great detail regarding the "why" behind this phenomenon.)