Because they WANT to capture the fact that a pitcher is not healthy.
I believe it's over the last 2 consecutive seasons, not just the last 2 seasons you played, and the categories Bedard is being judged on are games pitched, IP, Wins, W-L Percentage, ERA, and Ks.
Bedard is not making top 20% in many of those categories based on this season and last season. He's not an A and I can't see him being a B either.
So unless I'm wrong all we're talking about are his last 3 months' worth of starts. Now if you'd like to bet that Bedard's shoulder can hold up and he's a more battle-tested pitcher than someone like Fister, that's okay. My personal rating goes Fister/Bedard/Vargas right now, so if somebody wanted to give me the same deal on Vargas or Bedard I could see keeping Bedard and re-signing him. He seems to like it here.
But Vargas doesn't have name cache. It doesn't matter how good he is, it matters how good you can sell your team and your fans that he is (in large part). We traded for Rey Sanchez in 2003 at the deadline, and he was basically a .300 hitter for us. It was still viewed as taking the weak way out at the deadline, which it was, and demoralized the team.
When the Astros trade for Randy Johnson or Carlos Beltran, it means they're serious, and both their fans and their players know it. If the Reds trade for Vargas instead of Bedard, people will question how serious they are - including people in the locker room, funnily enough. Doesn't matter if they had equivalent WAR or pitching potential. MLB isn't the same sort of star-driven league as the NBA...until the trade deadline. Then name cache matters. Bedard has it, Vargas doesn't - awesome sunglasses promotion aside.
I'm not opposed to keeping Bedard, but it's my feeling that he brings more in trade value AND Vargas is several millions cheaper through 2013 than Bedard will be. We need both money and prospects to get where we want to go. And on the downside, if Vargas goes on the DL he's only running us a couple mil and we have internal options to replace him. Bedard will be an 8 mil a season pitcher minimum, IMO, if he throws 170+ innings this year the way he's been throwing the last 6 weeks.
He's not gonna make 5 mil this year - his incentives should he have that season are higher than that. Why would he take another pay cut? 8-10 mil a year is what I'd expect to pay for him.
I need that money for a bat, and since his trade value should be higher - for some silly reasons, but higher - than a Vargas or a Fister, then I trade him now. Fister I trade in 2 years after we make the announcers compare him to Seaver 97,000 more times and he has a top-20 ERA performance or two. ;)
And he wins some more games. The dude is 12-25 career - forgetting WAR/$ for a second, how do you convince the 24 other guys on your team that he can help them win when he demonstrably doesn't win? Erik Bedard is a 15-game winner who has led the league in Ks before and been top-10 in ERA twice. He has the resume that Fister and Vargas don't have.
Bedard goes.
~G