Time to address what another reliever would bring to the team.
Let's take Jason Frasor. 2.47 FIP so far, ZiPS has him at 2.96 moving forward. Toronto's usage has left him with a 1.36 pLI this year, which would rank second on the Rays, just ahead of Dan Wheeler and essentially equal to Chad Bradford. Thanks to those two factors, Frasor has been worth 1.2 wins this season. For comparison, Wheeler has a 4.31 FIP and 1.21 pLI but has only been worth 0.2 WAR.
Relievers are ultimately valueless in comparison to other positions. J.P. Howell has been awesome and he's still shy of 2 WAR. Last year he was awesome and finished with 1.5 WAR. We expected Cliff Lee to give us about 1.5 WAR heading forward, let's take a look at the amount of relievers who finished with 1.5 for the entire season over the years:
2008: 18
2007: 22
2006: 20
That means expecting anything close to that for someone like Frasor is ludicrous. If Frasor simply pitches 30 innings for the Rays at his current rate, he'll be worth about a win for the team, but here's the caveat: he won't. A) He's not going to pitch the highest leveraged innings as he did during Scott Downs absence, and B) He's unlikely to pitch as well. That means you should expect even less than a win from perhaps the best relief option available.
Someone like Frasor would help the Rays, but has no illusions of him being overly necessary.
There's an entire chain of events that could happen if the Rays acquire someone like Frasor that I didn't get into. Since he's only here for a year and a third, and Howell is entering arbitration, the Rays could save money by shifting Howell back to relief ace status and implementing Frasor in the 9th. Or Frasor could take some high leverage innings as the Rays ‘platoon' relievers based on match-ups. There's also the impact of having less Dan Wheeler (or whomever) in high leverage spots which plays a factor, and the price that Frasor would require in prospects.