1. If you want to be able to pull the radio out of the airplane to use it at fly-ins, you might not want to use the battery eliminator. That would mean you'd have to carry the battery pack with you (anyway) and switch it out when you took the radio out of the plane.
2. One thing that's very nice is having the jacks for the headset solidly mounted in the panel or a side panel. Beats having the floppy adaptors hanging around, and makes neat cord runs easy.
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4. As has been mentioned, "headset adapters" for handhelds handle the ELECTRICAL connection, but the impedances are off for the headphone portion. Most aviation headsets work, but they lose about 6 dB of sound energy. You can boost the sound volume by using a headset with 8-ohm speakers. I've got an expanded version of the Kitplanes article at:
http://www.bowersflybaby.com/tech/amplitude.html
Ron Wanttaja