Doesn't the unit show you altitude derived from the GPS signal? I have a GPSMap60Cx, without the compass and air-pressure altimeter, and it does show GPS altitude.

My understanding is that the air-pressure sensor makes it more accurate. GPS height errors are roughly twice horizontal errors, mainly because the geometry is always adverse - the satellites are always above you. If you are just looking at it for fun, it probably doesn't matter.

I'm not sure how much more accuracy air-pressure gives you. Aircraft use them, but I believe they get their accuracy partly through having a reference sensor at the airport which can adjust for local weather conditions. I have an altimeter in my watch, and it won't even display to better than 5m (15ft). Admittedly an airplane would have more sensitive sensors than my watch, but I don't know how much better the sensor in a domestic hand-held GPS unit would be.

The bottom line is that I got the 60Cx because it was cheaper than the 60Csx and I didn't think the extra sensors added much value.

Other than fun, why do you even need altitude? I gather it can help find your location on a map, if the map has contour lines. With a GPS you shouldn't need that anyway.