I guess tall baseball pitchers are better than short ones because the former have a better chance at playing basketball and becoming two-sport athletes.
Tall baseball pitchers are better than short ones only if the taller pitchers actually use their height. Also, other things must be equal. A shorter pitcher who has been blessed with a great arm will be better than a taller pitcher who does not have the natural talent. Sandy Koufax would not be considered a tall pitcher today. So, this generalization is only valid when all other things are equal between the two pitchers.
Some incremental advantage must be ceded to taller pitchers because the major leagues lowered the height of the mound several years ago. I think the year after Bob Gibson came home with a minuscule ERA of about 1.23. Nothing like this has been accomplished since the lowering of the mound, so pitching from a higher hill (relative position to the plate) had to have had some effect favoring pitchers.
Given the equalities, taller pitchers can use their height by striding out and getting a good push-off from their leg on the pitchers’ rubber. This places the release point a little closer to the batter. When a ball is traveling less than the 60′ 6″ distance from the mound to the plate, at speeds in excess of 90 mph, those few inches can make a significant difference to the eyes and reaction times of the batters.
Another thing a tall pitcher needs to do is get close to the ground on delivery. Besides adding to the pitchers stride, this puts the pitcher more on a level with the batter and effectively increases the strike zone. In the case of Roger Clemens, one can actually see his right knee touch the ground, at times, as he strides.
These are, of course, all generalizations. A shorter pitcher, blessed with a great arm can make up for a lack of height in many ways. He can keep the baseball out of the batters sight for a longer time by working on his motion. Giving the batter a shorter time to see the ball, and any spin that may be on the ball, places the advantage with the pitcher. Changing speeds and locations, knowing the weaknesses of the hitters and knowing the umpires all add to the effectiveness of the pitcher.
Remember, Greg Maddux has built a Hall of Fame career out putting the aforementioned factors into play in his pitching. He cannot be considered a tall pitcher among those who pitch professionally today.