Like many have said, UTM is much simpler to use and makes a whole bunch more sense for land navigation.

Also like others have said, pilots like lat/long. On most Garmin handhelds you can set one of the data fields in the trip computer to display Lat/Long even though you may have the GPS set to UTM. This allows me to look up my position in Lat/Long should aviation need it.

It is more complicated if given a Lat/Long coordinate I need to place on my map or in my GPS, but doable (set the GPS to the given coordinate system, enter the coordinates and set back to UTM). However, the biggest issue when two people are using two different coordinate systems is they are often on two separate map datums as well... there is at least one place in AZ where the difference between NAD27 and NAD83 is almost 800m... ask me how I know