I've never been convinced that guys who don't get on base all that often are really impacting the opposing pitcher all that much. But any aggressive baserunner certainly does split the attention.
It would be instructive to see the stats of Curt Flood, Bake McBride, Joe Torre, etc. in games when Brock did and did not play. The days when Brock didn't play might be too small of a sample, but it would still be interesting, at least...