"Keeping the ball in the park" is most definitely a skill worth having for a major league pitcher. The less homeruns allowed by a pitcher, the better his team's chances of winning the game. Preventing homeruns is especially crucial for pitchers who don't strike out a lot of batters. If a pitcher allows a couple of batters to reach base via base hit or walk, a homerun suddenly puts three runs on the board for the opposition. (That's why a high strike-out rate/low walk rate pitcher like Johan Santana can give up a long ball now and then - 22 last season - and still have such a low ERA; those homeruns often were given up with few or zero baserunners.)
But what about giving up singles, doubles and triples? Opposing slugging percentage takes all of these into account, in addition to homeruns. The only problem with slugging percentage is that it measures as equal a light hitter who goes 4/4 with 4 singles and a power hitter who goes 1/4 with a home run. Now, maybe these two players are equal sluggers, but equal power hitters they are not.
Getting back to the Johan Santana example, we can see how opposing slugging percentage could be deceiving. If Santana scatters 8 singles over 9 innings of a complete game shut out, his opposing slugging percentage for that game would be the same as if he gave up 2 solo homeruns over 9 innings for a complete 2-run game. While there was less power-hitting in the first example than in the second example, the opposing slugging percentage for each game would be the same.
Still, opposing slugging percentage does a good job of measuring the value of a pitcher. Obviously, a pitcher with a low opposing slugging percentage is more preferable to a pitcher with a high opposing slugging percentage. 4 consecutive singles in an inning likely will do just as much damage (if not more) than a single solo homerun.
To see how much power-hitting a pitcher gives up, opposing ISO can help. A hitter's ISO is his slugging percentage minus his batting average. In subtracting batting average from slugging percentage, ISO better measures a hitter's power by adjusting for the potential high-average/only singles hitter. (Not that there's anything wrong with that kind of hitter!)
When applying ISO to pitchers in the form of opposing ISO, you can get a better idea not only of the pitcher's propensity for giving up the long ball, but also for extra-base hits in general. Opposing ISO alone can't tell you the merits of one pitcher compared to another. A pitcher could have a low ISO, but nevertheless give up too many hits and walks. Meanwhile, another pitcher could have a high ISO, but give up hits and walks at such a low rate that the times he does get tagged for doubles and homeruns, the damage is minimal.
What I think opposing ISO can be useful for is a "quick and dirty" way of measuring how hard a pitcher is hit that's a bit more accurate than counting homeruns allowed. I also think it could be useful as an "internal" measurement of a pitcher's potential for success if he can cut down his walk rate, raise his strikeout rate, and/or learn to induce ground balls. (Then again, inducing more groundballs probably would lower one's opposing ISO, as groundball base hits tend to be singles.)
So, without further ado, the 2006 Cardinal starters' opposing ISOs (including Ponson's)