Further to George's point, a team's pitchers have defined roles. You want the starter to go six or seven. However, that's even started to change because as Al noted a week or so ago, stats bear out that a pitcher loses effectiveness the third time through the order; the batters have gotten a good look at him in their first two ABs. In addition, the benchmark for number of pitches thrown per game is around 100; it used to be 125. If you're a contending team, you don't want to wear out your starters by August. Verlander is an anachronism; the pitchers who can go eight or nine -- the standard 30-50 years ago -- are a dying breed, if not almost completely extinct.
In addition, the relief pitchers have defined roles. You need a bridge between the starter and the closer. In the last few years, the emphasis is on having a 7th inning pitcher and a 8th inning pitcher. As you don't want to overtax them either, the closer comes in for the 9th. It used to be that a closer -- then called a fireman -- would come in the middle of an inning when there were runners on base. No more, for the most part; that's the job of the 8th inning pitcher. Closers now come in at the beginning of an inning.
Assembling an effective bullpen is probably one of the hardest -- but most important -- jobs for a GM. You cannot simply go with the pen you used the year before. It doesn't work that way for some reason. A pitcher that may be effective one year may be terrible the next and vice versa.