Kelley is 28 years old, a veteran of 120 relief appearances over four major league seasons and is signed for $935,000 for 2013 in his second of four arbitration seasons (he was a Super 2 in 2012). He has a career 3.52 ERA over 128 innings, including a 3.25 mark in 47 major league appearances in 2012. He walks around 2.7 batters per nine innings and strikes out 8.6, but also allows 1.3 home runs per nine.
Kelley's major league numbers have been good, but his minor league numbers are nothing short of phenomenal. He has a 1.78 ERA in the minors over six professional seasons, striking out 10.2 batters per nine while walking just 2.9. He made 14 appearances in AAA in 2012 and had a 0.90 ERA.
Could he help this team? Almost certainly. Kelley throws relatively hard (his fastball sits around 92-93), has a good slider and throws strikes. His tendency to give up home runs might make him a poor fit for late inning duty in close games, but the Brewers could certainly do worse in middle relief.
It's also worth noting, though, that Kelley has never pitched a full major league season. His 47 appearances with the Mariners last year were a career high, and he's never worked more than 46 innings.
What do I think the Brewers should do? This is a tough one. If the Mariners put Kelley on waivers he'll very likely get claimed before the Brewers get a chance: The entire AL and nine NL teams would have to pass on him for the Brewers to have the top waiver priority.
So, for the Brewers to be certain to get him they'd probably have to work out a trade. I have no idea what the Mariners would want in return, if they'd be willing to give him up for something as small as cash considerations or if they'd want a prospect or major leaguer.
At any rate, if I were Doug Melvin at the very least I'd pick up the phone and ask.