If relief pitchers were any damn good, they'd be starters.
That's a bit of an exaggeration, of course; these days some relievers are groomed to be short men even in high school. They pitch short outings throughout their minor league career and by the time they reach the majors, asking them to throw two or three innings would be a reach.
The majority of pitchers in the seven-man major league bullpen, however, were starters at one point in their career. Some lacked the stamina to pitch deep into ballgames, others failed to master a third pitch that would get batters out the second and third time through the order. A few others (Kerry Wood springs to mind) simply couldn't stay healthy as a starter and found success in more limited roles.
Therefore, it isn't really fair to expect relievers to be, well, as good as starting pitchers. They are going to have bad outings, bad months, sometimes bad seasons. Closer performance, especially among the elite members of that fraternity, tends to be consistent. Other than closers, you can probably count the number of consistently effective middle relievers in MLB on two hands. Those few relievers who can put together solid years over and over again throughout the course of their career generally "graduate" to closer status anyway.
On a day like today, it's easy to be angry. Joe Smith, Pedro Feliciano and Aaron Heilman spit the bit against a second-division team and it costs the Mets a badly-needed victory. It's almost unfair to be angry at them, however; if Smith, Feliciano and Heilman were so good that they could consistently put up scoreless innings in big situations, they'd be closers (or in Heilman's case, a starter). Consistently putting middle relievers into high-leverage situations is a recipe for disaster.
The answer, however, is not for Omar Minaya to run out and sign a bunch of middle relievers who were effective in 2008 to three or four-year deals. That's how you end up with pitchers like Scott Schoeneweis taking up space in the back of your bullpen. The Orioles were the latest team to try this short-sighted strategy after the 2006 season; since then Danys Baez has blown out his elbow, Jamie Walker has missed significant time this season to injury and Chad Bradford has already been traded. Baez and Walker will cost the O's $10 million in 2009, regardless of whether or not they actually pitch.
Reliever performance is volatile and will generally fluctuate from year to year. The good general manager knows this, and constructs his bullpen with a series of pitchers he has no long-term commitment to. Every year, the best bullpens are generally those that catch lightning in a bottle with three or four guys, not those featuring three or four relievers on multi-year deals based on the work they've done in the past.
The 2008 Mets, poorly constructed in so many areas, do not have a poorly constructed bullpen. Only Schoeneweis and closer Billy Wagner are blessed with multi-year deals; everyone else is singing for their supper in 2009. Heilman in particular is costing himself a lot of money. Duaner Sanchez, after such a wonderful start to his season, may be doing the same. Only Schoeneweis is basically impervious to all of this - he has a guaranteed deal in 2009 and will be run out there 65 times no matter what happens.
So deal with it, Mets fans. If your starters only go five or six innings a night, your relievers are going to get three or four opportunities to blow the game. Don't be so surprised when it happens - just be happy that these guys won't necessarily be here forever.