Last year the best baserunning team was +10 runs, the worst was -17. In terms of just base stealing (as opposed to taking extra bases, etc.), the range was from +5 to -6. So, speed on the basepaths might be worth 3 wins between the best and worst teams, pure base stealing might be worth 1 win. Then…speed will help on defense, too.
I’ll leave it to others to decide if these numbers suggest steals/baserunning is important/unimportant.