Okay, I'll bite..

What happened?

I usually use MGRS due to where I get my maps (thanks old USGI buddies).

But as has been mentioned, I also wind up doing conversions. Many cell companies use different systems, but I do work for all of them. I always ensure I can work within all the systems. When we move into an area for work, I contact the local FD, EMS, PD, and Air Ambulance folks. One of the items I get is how they want their directions and GPS info. Cell sites are given addresses, but the address usually has very little to do with where it actually is, and how to get there.

I've seen the difference in datum as well. In the case where we found it- .10 mile in the area of a group hike.

From the air that's not a big deal. By foot in the woods - it matters.

I like UTM because the grid lines are frequent enough on the map you can find where you are. USGS quads don't have frequent enough lines to suit me. And then you are stuck interpolating between intervals of 2 minutes 30 seconds per line.

I've written about this on my website-
http://coloradowreckchasing.com/UTMesHiking/UTMeshiking.html

A lot of what I do has to do with ELT's. AFRCC gives us the coordinates as Decimal Lat/Long. Truth is as poor as the fixes are it is more of a general location to start the search, that a hard fix.

For most other uses UTM is the way to go.

I 'grew up' with UTM and prefer it. Problem is most geocaching and most run of the mill guidebooks run off Lat/Long.

While I prefer UTM, I am being forced by convenience to use Lat/Long system.

On top of this, maps come in so many different scales now that it's almost impossible to find a map at the last minute or with trail detail that matches the scales on my UTM card or whats on the base of my compasses.

I too ONLY use UTM-I tried the lat/long thing, and I simply cannot convert. I blame the Army...
Its hard to find topos in UTM, but I have a program that was given to me a few years back when I was on S&R that allows me to print maps in any scale I need, which makes it most convenient.

Another variable is who you NEED. My store is in Idaho. So I am interested in using the grid that will get me the most accurate and fastest response. Local agencies all use NAD 83/84.

I keep a scanner on at all times. I listen to all of the calls for rescues, accidents, marijuana fields and collared wolves. In most cases the people on the ground cannot communicate to the 911 operator or the helicopter on the way. In Idaho DO NOT USE UTM OR DEG/MIN/SEC. Do not pronounce the decimal point! Only pronounce the numbers - talk slow - they are writing.

The LIFE FLIGHT, NATIONAL GUARD, SEARCH AND RESCUE, DEA and the 911 dispatch all want you to read the location as -

"The accident is at 4396679 by 11618880"

I keep a window open in http://mapper.acme.com/

ACME comes up as -

N 43.96679 W 116.18880 MY FRONT DOOR

N 44.08265 W 115.82877 A WOLF DOWN

UTM for me. Much easier to work with. Only UTM downside I see might be for large area navigation, such a airplane pilots need. Lat/Long is more common for those folks. With UTM, you can look at two gridlines (either NorthSouth or EastWest) and determine the distance between them in meters. Simple math. You can't do that with Lat/Long. The distance between lines of Longitude differ, depending on what Latitude you're at.

I will use most any projected coordinate system to get along. Last few years while on continent I used the following:

NAD 1983 State Plane (FIPS by region). Sometimes use the 1927 State Plane system. For projection, I prefer the Lambert Conformal Conic Central Meridian of -91.33333333 with Standard Parallel of 29.30000000 & 30.70000000. My latitude of origin is 28.50000000. Geographic coordinate system of GCS North American 1983. Datum: D North American 1983.

I work in either meters or international feet. Sometimes work in US feet in north America. Back in the day, cotton rows.

Make real time adjustments for the wobble of the earth. In this regard, stay locked to particular stars for tracking.

UTM for land nav, Lat/Long for air or sea (actually any activity that will take you more than about an UTM zone of six degrees longitude.)

As others have said, UTM has a finer grid, easier distance measurements and easier callouts (unless you're straddling two zones, just need to read the easting and northing and you can drop the last two digits for 100m accuracy.)