It would depend on what you mean by a "good" pitcher. Obviously, Schilling, RJ or Pedro are worth more than any closer, and you wouldn't trade them for closers anyway. If you mean an upper tier starter like Morris, Millwood, Wood, Prior, Halladay, Brown, Perez, Oswalt, Hudson, Mulder, etc., I would say that you could trade one of them for one of the elite closers (Smoltz, Gagne, Foulke, Guardado, Wagner, Percival or Benitez) or possibly a couple of mid to low range closers (like Julio, Politte, Mantei, Williams, etc.). If you are trading outfielders for closers, you'd probably have to trade a top 20 or so outfielder at least to get one of the elite closers and probably a top 40 outfielder to get a mid to low end closer.
Yes, closers do help you in only one category, but they command alot in trades, because there is a limited pool of closers available, with there only being 30 MLB teams, and some of the teams not having any viable closer options (Cards, Padres, BoSox, Tigers). In a 12 team league, that leaves only an average of 2 closers available for each team. It's not like outfielders or starting pitchers where you can easily go to the waiver wire and find adequate replacements, and that's why you have to pay a premium to get them.
First check, and make absolutely sure there is nothing available on the wire for low range closers that people have overlooked. If not, in a 12 team 5x5 league, you probably need to finish around the middle of the pack in saves to avoid killing your chances. Since an average closer gets 30 saves, that would mean you need to have a total of around 60 saves. You could get that either by getting 2-3 low range closers or by making a deal for a Smoltz or Gagne that will get you 50+ saves.