This is true. When you are on base more often, you give yourself more opportunities to knock in more runs. Cubs were awful last year with runners in scoring position to begin the year....but they finished the year top 10 in runs scored partly because they were on base so often and had more chances to score.
http://www.espn.com/mlb/stats/team/_/stat/batting/sort/onBasePct/order/true
13 out of the top 15 teams in OBP had records above .500. And the 2 that don't (Texas & Cincy) were bottom 10 teams in pitching.