*Note: This is true for all pitchers as opposed to just starting pitchers, but most people seem to know not to use this for relievers. Also, most relievers don't accumulate enough wins or losses in a season for someone to want to use their W-L record.
A major factor in a starting pitcher's W-L record is how much run support he gets from his team. This is completely out of his control if he's not batting, and most pitchers don't hit well enough to be a major factor of run support even when they are batting.
Unless a starting pitcher pitches a lot of complete games, which is very rare in this day and age, the other reason a pitcher's W-L record is virtually worthless is his bullpen. Bullpens can blow a starter's leads and cost him wins. Or if a starting pitcher leaves with a lead after just 5-6 innings, bullpens have to do the work for at least 1/3 of the game to preserve a win.
I'll give you a couple of extreme examples. A starting pitcher could pitch a complete game and give up 1 run (even an unearned run), but lose 1-0. A starting pitcher could pitch a crappy outing, but a good bullpen and a lot of runs from his hitters can bail him out from a loss. He could even get a win in that situation if he pitches at least 5 innings.
You could also make an argument that a pitcher's defense behind him is another reason not to use W-L record. I wouldn't use that argument because defense also affects stats that are a lot more relevant for a starting pitcher like ERA, ERA+ or opponents' AVG and SLG.
As I have often said, I'm no sabermetrician. If you're looking for more advanced stats like VORP, I'm not your guy. However, this means any casual fan can understand the stats I use to show how good a starting pitcher is, like:
- ERA
- ERA+. If you're not familiar with ERA+, it's park-adjusted and it's rated the same way a person's IQ is. 100 is average, > 100 is above average and < 100 is below average.
- Opponents' AVG/OBP/SLG
- IP, H, BB, K
If you've seen this post before, you already know why I don't use ERA or ERA+ for middle relievers.
