Interesting subject (to this ex-pitcher), so I'll chime in. I'm in agreement with both of your underlying points: the invaluable weapon a consistent basestealing threat is.
I've said it to everyone who's ever asked me: name one player from the past 50 years you'd want to build a team around, if you suddenly became a franchise owner. Answer was, still is, and will be so long as I know a baseball from a tennis ball: RICKY HENDERSON.
Nothing, but nothing, will unsettle a pitcher like a bonafide threat to take off, and make it, from first at any second. And with that "C'mon, MF'er, just try to pick me off!" look of his, he multiplied his effect on the pitcher x-fold...believe me.
And you needn't ever have pitched, at any level, to know what happens when a pitcher becomes unsettled/unnerved/has-his-mind-messed-up:
Fat meatball pitches to the hitter. Bases on balls. Errant pickoff throws. Balks. And bang, there goes a rally for the opponent.