The utility of a transceiver
I am certainly not advocating that everyone needs a transceiver, in fact, the TT4 platform ( transmits and decodes) versus the TT3 platform ( Just transmits) requires a significantly higher degree of APRS Gung Fu. By adding the receiver, you can send text messages, "talk' to other stations with a keyboard and LCD, computer, or some GPS receivers, and of course, actually see the position of other APRS trackers and objects ( Digipeaters, weather stations, etc) The transceiver can act as a digipeater and a tracker, but you don't have to do either or both....you have a choice, in fact, the array of choices with a transceiver becomes so great that you can create a nightmare for yourself. One problem that occurs in an airborne transceiver is that the receiver is capable of picking up stations directly and clearly out to the horizon ( which we search and rescue guys know, can be pretty far away) Ordinarily, this means that your transceiver, which is designed to be "Polite" will not transmit until it "hears" a clear channel. At altitude, it will never have a clear channel, since it will be picking up stations quite well for about 15% more distance than the optical horizon. This means you need to make it a bit "Deaf" or it will never transmit. Like the transmitter-only, this means that you can't go hog-wild on how often you transmit.
Also, acting as a digipeater, if you set the digipath too wide, it will store and forward APRS transmissions from other stations, and at altitude, this can wreak havoc, since not only can you receive stations from hundreds of miles away, other stations can also receive your digipeated signal from hundreds of miles away.
There are sneaky ways of getting around this. For air and ground Search and Rescue, or special events when you really want specific coverage without flooding the whole continent with redundant traffic, you might set your unit to only digipeat stations that have addressed their calls to a specific alias, like "SAR" or "Temp" and ignore the others. If you within range of a known-I-gate/digipeater owner, you might convince the operator (if he is the aviator type) to only digipeat received signals on your own "secret" frequency, while still sending your own position reports to the internet so you can see them online on aprs.fi, etc.
By connecting a transceiver via bluetooth, you can run software on your Droid phone or black box with software like Aprsdroid, so you don't have to add more gear to your panel. Personally, I like to marry TT4 or Kenwood based transceivers to the Avmap G5 or G6 display GPS receiver. This receiver will display APRS icons, so a Plane will show up as a plane, a truck a truck, a balloon a balloon, etc. The Avmap will also allow you to send text e-mails from the touch screen, or open up APRS weather stations with a weather station graphic display.
The MTT4B will make a great ground station digipeater for areas where there are holes in APRS coverage. Pretty much all you need to do is add a base antenna and a 12 Volt power supply, you don't even need a GPS. If you marry it to a PC, you can use it for the front-end of an I-Gate, a Weather station, telemetry station, etc., but this is not for beginners without direct support from experienced Hams.
Allen