Whether to upgrade or not is an individual decision. I suppose if you absolutely *must* have the latest map data, than upgrading every year is appropriate. We used S&T 2005 until just recently, and found that while some of the data was stale for the most part it met our needs, especially since we downloaded the regular road construction updates.
Terry is right, though, that Streets & Trips 2010 has some *really* neat new features like, for instance, the ability to show/hide pushpin sets (great for keeping your map uncluttered with unwanted pushpins). We upgraded only because our gps was dying, so we bought the new version for the gps not even knowing there were great new features. We're sure glad we did upgrade, though... we *love* the new features.
Hope that's helpful...
Regards, Chris