Based on a national strategy that in essence says we will take the fight to the enemy versus let the enemy come to America, the U.S. military is expeditionary in nature. Since there are lots of different types of enemy forces in different climates and terrain, we have to have a broad mix of capabilities ourselves.
The trick is to not get so specialized that units become "one-trick ponies"--only capable of fighting in one geographic area against one enemy force.
Our current mix of foot infantry units with the strategic and operational forced entry capability of the 82d Airborne and 101st Air Assault coupled with the "heavy" force Bradley/M-1 tank mix is pretty good. However, we've been missing a "medium" weight force and that is what Shinseki gave us with the Stryker Brigades.
I agree with many that the M113A3 was on the shelf and ready, but with either vehicle, the purpose of the brigade would be the same--get there faster than a heavy force with a lot more firepower than foot infantry.
Further, after deployment, in areas where the ground is reasonably trafficable (most of the world), the Stryker Brigade will provide the advantages of vehicle mobility coupled with much more foot infantry than heavy forces carry--the ratinale is not simply strategic mobility--the Stryker Brigade also fills a tactical need.
As for fighting tanks--Stryker Brigades would have no problem defending (defending an airhead is one of their primary tasks) against heavy armor units. Offensive ops, except against an enemy with a very limited number of tanks, would be very problematic, however.
Meaning offensive ops against a primarily "heavy" force.