First, let's be clear about the question:
Are we asking
1) Does a particular player's power and average improve if he increases his walk rate; or is it
2) Given players of identical ability from a rate perspective (per AB), is a higher walk rate better?
Regarding #1 - I ran correlations on 4 years of data, and of course I can't find them now. I do recall the correlation between BB rate and batting average is extremely low - 2-2.5%. That is without even subtracting out IBB, which reduces the correlation even further. I really need to find the data, but what I recall is that players with higher walk rates also have higher strikeout rates - not really that surprising as they take more pitches. So any gain from improved "selectivity" is largely offset by decreased contact rate. I do seem to recall that higher BB rates correlate with higher SLG, but even if true you are left with the chicken-egg question. Correlations do not imply causality. I will hunt around for the data next week.
Regarding #2 - I did an analysis comparing 2 identical average players in a current day AL-only 10 team league environment, assuming the only difference is that player A walks 30 times more while player B gets 30 more AB. With those 30 BB, player A will score 8.5 runs and steal about 1.5 bases. Player B, with his 30 more AB will produce about 1 HR, 4.5 RBI, 3.5-4 Runs, and .3 SB.
Netting it out, Player A has an advantage of 4.8 Runs and 1.2 SB vs. Player B with 4.5 RBI and 1 HR. So the average player shows an advantage from the BB, but it is very slight. All the BB in this scenario are assumed to be non-intentional.
Note that relatively few players are average in SB, which is really the key stat in the analysis. If the player isn't much of a base stealer you are better off with the AB than the BB.
In shallower leagues the equation may be quite different, but I haven't done that work up. Players are significantly better hitters, so they will produce more HR with those extra AB. Runs and RBI rates will both increase, though I would guess RBI would go up more. So in shallower leagues, I wouldn't be surprised to see the AB become equal to or greater in value. Of course the entire value question also becomes tied to how you value a SB, which is a contreversial subject.
So the simple answer is that if you have a base stealer you want him to BB, and if not you want him to get more AB.