Is there any reason to switch to the Giant Aircap headset expander? Any reason not to? I ride a Giant road bike with OD2 carbon steerer. It was originally built up with a standard expander plug, but was supplied with the glue-in Aircap expander as well. Im not sure whether to use it and there is very little information about these online.

I've only been able to find two things about it so far. Apparently it's a bit lighter, but I couldn't find how much lighter. It seems to also be a lot more hassle if you install it then want to cut the steerer again. If that's the only downside to it, then I'm happy to use it because my steerer is already cut down to have no spacers underneath. Is there anything else that should affect my decision to use this or not?
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).