In my opinion having a compass with adjustable declination is important because depending upon where you're located in the U.S. magnetic north can be as much as +/- 18 degrees off from true north. The Silva Polaris/Classic/123 compass has an aid drawn on it to help do the adjustment with each use, but with that I think mistakes can happen, especially if stressed.

Magnetic declination is more extreme at the northwest and northeast corners of the continental U.S., and close to zero near the Mississippi River. It's always a good practice to look up the current magnetic declination before you head to an area (it varies slowly over the years), and pre-adjust your compass' magnetic declination. Also make sure your GPS is set to display true north, and know how to find the true north on your paper maps.

If going to a global area for which the compass isn't designed for, then having a global needle necessary. If, for example, you buy the compass in the U.S., and have no plans to use the compass outside of the U.S., a global needle is not needed. On some Suunto compass models I find the non global needle much easier to use than its global needle counterpart (the red/white coloring is much more clear).