I'd say it takes up about 50% of my MLB bets. in fact I probably on bet that and 1st five innings as you can't handicap later in the game, in who the manager is going to decide to bring in from the bullpen and what not.
I know some people look at it this prop and say "its stupid the best 3 hitters are up in the line up for both teams"
but this is baseball and pitching dominates, if you choose the correct spots and correct pitchers, you will come ahead on these more often then not, I've made a killing on em the past 2-3 years.