There is no absolutely reason to replace it if it is still good. I have never replaced a battery unless it needs it. And, in our cold winters, our batteries get one hell of a workout, but they will be good for a long time unless you kill it a few times by leaving the lights on or something.
So, if it is starting to crank a little slow, replace the battery, otherwise leave it in. No point in needlessly disposing of batteries in working order...after all, aren't all of us SUV drivers environmentally friendly? ;-)