As a former professional player who spent 10 years coaching high school and youth baseball, I can tell you the difference is night and day when it comes to stretching. I know it has to do with lack of experience and understanding, but with as much technology that we have today plus the "live look-ins" from MLB Network during pregame, I feel like it's just an excuse. Young players dream of playing MLB and mimic their defensive and offensive movements, but the fail to mimic their training and preparation.
Baseball is a game filled with more downtime than play time. Any running is going to be in a span of 15-20 seconds at max, to about 6-7 seconds on average.
Imagine Usain Bolt hopping up to the starting line for the 100M run without warming up and stretching his muscles! he wouldn't last 25M!!
Without a doubt, stretching is not only worth it, but you should not consider yourself giving your best effort unless you're stretching. Working out and practicing are keys to improvement, but stretching pushes you past an invisible barrier into realms unknown.
Make it a routine: get up in the morning and do a single stretch. I always start my day with the Pigeon Stretch (click here). This loosens my hips and my glutes and kinda helps me wake up.
Try it tomorrow and see what you think!