Baseball scouts seem to value taller players because the increase in height of the average player has exceeded the national average. http://www.beyondtheboxscore.com/2011/4/19/2114631/the-changing-size-of-mlb-players-1870-2010
They seem to value weight and muscle mass too. Average weight has increased at an even faster rate than height. http://sportsologist.com/baseball-player-height-and-weight-by-year/
Unless the scouts are wrong, taller and heavier make better players. Of course that isn't proof that players are better than they used to be, but it would likely be an indicator. Obviously I think it is.
I have to think natural selection is also having a strong effect on height. Though studies have shown that the more affluent (accounting for better medicine and nutrition) countries have gotten taller than poorer nations, those nations also have more access to a national and worldly press that would seem to present taller and stronger as more attractive than shorter and weaker.