The Astros are in the hunt for Iguchi. If they do sign Kaz, they aren't getting rid of Burke. It will be "may the best man win", or basically create a little competition for the job. The loser gets the backup job, although Burke is more versatile. If they sign Iguchi, it's different... Burke definitely becomes the backup at that point.
Kaz Matsui doesn't get the starter job automatically if they sign him and I would be very surprised if this is what they intend to do. And a one year plus a club option is all I'd spend on Kaz. Another thing, perhaps they feel closer to signing some relievers (Linebrink? Affeldt?) or a starter (Lieber? Wolf?) so Kaz fits the budget moreso than Iguchi if they need to pay a pitcher.
Like I said, if they sign Kaz, it probably is insurance for Burke in case he proves in spring training he's not ready (yet again) to take a full time job.