The Cards must think his skillset will age well then. Either way, the extra two years would give them a bit more time to develop a 3Bman in the minors. Right now, the only notable guy in the system is Patrick Wisdom, who has the glove but lacks the contact skill.
Considering what his particular skills are, they would probably be right in thinking that. I'm more concerned about injury risk however, which starts increasing substantially into the 30's. I'd like to think about it in terms of expected utility. For example:
Player A is projected to be a 5 win player, but has a 60% chance of missing more time than expected. Player B is projected for a 3 win season but only has a 20% chance of being seriously injured.
Player A's expected utility would then be 5*0.4, or 2 wins. Player B's would be 3*0.8, or 2.4 wins.
Those example numbers are extreme, but hopefully highlight my point. I'd rather go with younger payers then older ones, even if the older ones are better players.