Is there any reason to switch to the Giant Aircap headset expander? Any reason not to?
Many people tend toward a "if it ain't broke, don't fix it" mindset with bicycles.  I think you already understand the main differences (weight, inability to shorten steertube after installation).  The only other immediate difference is cost (money for a new expander, versus the one you already own) and labor.

One other difference may be that if that fork is somehow damaged/destroyed, you'll likely lose the glue-in, however, you have original as backup in that case.  Really it's just a case of you deciding to spend the money to save a few grams (that's assuming the glue doesn't make up the weight difference and make it the whole thing just a marketing gimmick).