Defensive issues? What exactly does Spezza do that is lacking?
Play defense at all competently in either the neutral zone or in the defensive zone?
Have you watched Spezza? Look I like the guy and all but he's got some flaws, some pretty serious ones which is why Ottawa is considering moving him. And one of those flaws is his...let us say disinterest...in playing defense.
I don't doubt the guy's compete level, but he obviously prefers to expend his energy in the offensive zone which is part of why his play on the other side of the ice is often lacking. And he's just not very good at it. He plays one way when the puck is being driven forward and is a lot more...timid when it is coming back.
As for injuries, you realize he has played more games than Backes or Steen did last season right?
Couple of points here:
In regards to the injuries themselves: Yes Spezza played in more games, but he was not terribly effective in more than a few of those games due to his groin injury. I understand playing through pain, but there were games where he was simply ineffective. I know it made his already shaky defense pass bad and go right to abysmal.
We still have the fact that he missed the entire previous season with a back injury. True, lockout and all that but he still missed it.
In regards to the amount he played vs Backes and Steen:
With Backes' play style that is to be expected, he is going to miss games simply because the body cannot take that sort of pounding for a long period of time. I could get in to why Backes should be starting to alter his play-style but this really isn't the best place.
Steen we know is an injury risk and the Blues frankly overpaid to keep that injury risk on their team due to him being the best 2-way forward on the team. I'm not sure it's something I would have done, but I understand why they did.
Regardless, if Steen were to be coming on to the market right now I'm not sure I would want the Blues trading all that much for him either. Certainly he has impressive potential and his game has done the usual Swedish late bloomer thing, but those concussions are worrisome.
(tldr)
So to sum up:
The Blues should have the following concerns with Spezza:
1) Injury history: How much are his previous injuries a concern, in particular for a player on the wrong side of 30?
2) Defensive play: While the Blues cannot and should not expect him to magically become a 2-way forward, can Spezza play enough defense to continue to play center in the Blues' system? If not will the Blues be willing to adapt their system enough?
3) Reaction to the Western Conference: The West, in particular the teams the Blues play, tend to be very physical teams. Spezza's health as well as his style of play may or may not mesh with that particular issue. If not then we have a player who becomes increasingly gun-shy which you do not want in your #1 center.
4) Cost: How much are the Blues going to be required to give up and can they live with removing those pieces?
5) Re-signability: If the Blues give up a decent price for Spezza and he turns around and either demands 8+ for the next 7 seasons or wants to go to a different team how much will that affect their plans?
6) Other possibilities on the Market: Obviously Stastny should still be target #1. His offensive capabilities are not far off of Spezza's (the current Spezza) and his defensive play is head and shoulders better. Additionally since the Blues have a team set up to where they plan to roll at least 2 primary lines, the other center is not required to be a Malkin/Crosby/Stamkos/Getzlaf and any good #2/marginal #1 should fit the Blues fine.
I like Spezza and all, but thinking he is the only other option to Stastny (or, dog forbid, preferring him to Stastny) is a bit short-sighted.