This is almost certainly the least suspenseful contract jockeying of all time. Trever Miller is not a bad pitcher, and I don't mean to act like he is, but his inconsistency from year to year, when combined with his low upside, makes him indistinguishable, to me, from any of the minor league fliers that the Cardinals could take heading into the season. He might help you out some, he might hurt you a little, but he's probably not going to put you on his left arm and say "don't worry about it, boys—this is my team now" anytime soon.
Over his last five years Miller has had two good seasons, 2006 and 2004, and three marginal ones. He's 35, so he might lose it, but he's a lefty, so he might not; he's got good control sometimes and bad control some other times. I don't see any particularly brilliant way to suss out whether or not his next fifty innings will be good ones or bad ones, short of counting the number of left-handed hitters he's slated to face in each one.
It's proven less true the last few years, when the Cardinals have increasingly signed free agents and developed prospects to fill the back of the pen, but the Cardinals have a history with that Beyond the Boxscore perennial, freely available talent. Through scouting and luck the Cardinals occasionally, after going the minor league route, come up with a useful player. When they don't—well, as Bill Pulsipher would vouch, they either make the team and disappear or end up a Spring Training memory, harming nobody in particular.
So let's say that, in addition to Trever Miller, the Cardinals were scouting a reliever named Talent Available Freely—you can name him whatever you want, but my reliever is Moe Szyslak's acquaintance's brother—who put up the following lines over his last four seasons.
T.A. had some command problems early in his career, much like his brother, and sometimes he's more fit for low-leverage work than anything else—just like Trever Miller.
All of the players here cost the Cardinals nothing, in baseball terms, and all of them were effective for at least one season. Of course some of them bomb out, but that's how this sort of thing works; all of the players who aren't guaranteed jobs fight it out, in some weird pseudo-Darwinian fantasy, and the good ones get a season's worth of numbers.
In 2005 Randy Flores could've been usurped by Bill Pulsipher, Carmen Cali, or Gabe White; any of them could be making up a year of the composite instead of the Bellflower Bomber. The individual players are less important when there are so many of them, and in relief pitching more than anything else it seems like quantity is worth far more than quality when you're looking for just-average types. (Can anyone name a consistently "average" relief pitcher, off-hand? I tried for a while but I couldn't.)
It would be great to have a solidly "good" lefty reliever in the pen, so that the onus isn't on these candidates, the Charlie Mannings and Ian Ostlunds, to do anything more than pitch the Aaron Miles innings, but I'm not sure how Trever Miller accomplishes that. He's probably an improvement on any one lefty relief candidate currently in the Cardinals' plans, but I don't think he's an improvement vs. the field.