Can't have it both ways
Hennessey and Correia probably will turn out to be good 3/4 and 4/5 starters, respectively, eventually. They have shown that when they are on, they can be good starters. I recall one baseball expert saying that once a player has displayed a skill, he owns that skill. However, young players, like Hennessey and Correia, frequently are trying to figure out how to be "on" on a regular basis, how to harness and utilize his skills properly.
Hennessey is a good example. When he has been good, he has been very good, shutting down the mightly St. Louis offense, for example, in a quality start. However, as the nursery rhyme goes, when he has been bad, he as been very bad, as he just showed in his last start on Saturday, giving up 8 runs in only 4.1 IP. In fact, if you go through all his starts, he has displayed this Jeckyll and Hyde tendency.
Correia has been a little more consistent but closer to the mean. Nothing flashy or dominating but good games pitched, for the most part, with one bad pitch separating the good starts from the bad. That is probably why Hennessey got moved back to the bullpen when Reuter was returned to the starting rotation and Correia is now the #5 starter.
Ellison has been another good example. Over the season, he has had his ups then his downs then more downs, dropping in OPS from 1.201 to .733 to .660 to .503, month by month. Grissom is coming back at the right time, Ellison should sit a while and regroup and provide a spark from the bench.
If Sabean is not using old, experienced vets, he is basically forced to go with young, inexperienced players because young experienced vets are costly, either in terms of salary or prospects traded for him (see A.J. Pierzynski). And young, inexperienced players do not have consistently good performances unless you really got a gem. They are hot and cold, all the time. And it is hard to fight for a pennant when you have players who turn cold regularly like most youngsters do. Hence why Sabean goes for the grizzled vets who have a body of performance data that gives a clue as to how the player will perform if playing regularly.