Would it built a small amount of strength?
Not really. There are different kinds of strength (i.e. limit strength thats acquired by lifting heavy loads for low reps, strength-speed such as o-lifts, speed strength such as jumping squats, etc.) but 100 reps just builds up some endurance for very light weight.
I picture boxers who throw 1000 punches with 16 oz gloves as weights. Overtime they develop strength as well as endurance specific to that body movement.
But you have to be careful about that because force generated (i.e. power) also needs speed, and using implements that slow you down will result in technique changes and a slower movement. This is why heavier baseball bats fell out of favor as a training tool in baseball.
I know it inst the optimum way and many other exercise can come in to play. But wouldn't 1 set x 100 reps still have some strengthening affect on the body parts composition.
What does this mean? It will only burn calories as Shelby said. Its not going to partition or spot reduce a certain area.
You can reach failure with very light weights if you do the movement enough times
So? You dont need to (and oftentimes should not) go to failure to develop athletic capabilities such as speed, power, strength, etc. By your statement, why not go grab a 1lb pink DB and curl it 10,000 times, see what happens.
Btw, theres also repetitive stress and overuse issues from doing high reps like that too often.
It certainly works for that physical freak.
He was a freak before doing 100 reps. This just shows that the genetically gifted will excel no matter how retarded their training program is.