The Blues could always use one or two extra defensemen in the stables. As we saw last season, as every defenseman except for Eric Brewer missed time with injury, the quality of call-ups is important. The thing that the Blues might want to avoid this season is having said call-ups play for half of the year. It's swell that Nikita Nikitin got 41 games of experience last year and really blossomed, but he was the exception, not the rule.
The problem the Blues have here this season isn't necessarily defensive depth (though a few spare cogs never hurt anyone) but what to do with Nikitin? He's presumably the team's 6th defenseman. You don't take young talent like him, and make him munch on pretzels in the pressbox for 3/4ths of a season. The Blues are more than likely going to have to add a 7th defenseman since they decided to not extend Dave Scatchard a qualifying offer.
A cheap alternative to the UFA market is to offer a contract to either Tyson Strachan or Nathan Oystrick, both pending UFAs and both capable 7th defensemen. If they want to look at the market, and wade through some of the bigger names (no, Blues fans, the team won't make a go at Tomas Kaberle or Anton Babchuck - or even Brent Sopel and Paul Mara), there are a few capable, older defensemen out there who could be role guys.
Tops of this list is probably Shane Hnidy. Inexpensive - last year's cap hit was just a half-million dollars - he brings years of NHL experience along with a Stanley Cup ring. Unfortunately, Hnidy's been injury prone, missing a decent chunk of time last season with a leg injury and a shoulder injury sustained in training camp. He played as many games in the playoffs as he did in the regular season.
Another possible addition could be to bring Jeff Woywitka back from Dallas, where he had 2 goals and 9 assists in 63 games played.
Freddy Meyer would be a solid choice for the Blues, but he's under contract to play for Modo in Sweden for next year. Meyer missed the last third or so of the Thrashers' season last year after suffering a concussion in a game against the Calgary Flames and collapsing center ice. No word's been released on his condition or if he's expected to be in playing shape for this coming season.
It's slim pickings for non-regularly playing defensemen this year. The Blues might as well toss a contract at Strachan or Oystrick.