Some people have two gains with it: The first, of course, is 2wd low range.
The second is, for control freaks like me, anyways, that it wrestles control of the system away from the computer and puts it back in the hands of the driver. I haven't been a fan of the Control Trac algorhythm since the first time I drove it in a snowstorm. I pulled out of my condo complex, and intentionally put it into a 4-wheel drift to get the feel of it. The timing of the C-trac cycling the front drivetrain in and out put it into a scary and almost unmanagable fishtailing, power-on situation that wagged the tail of the Explorer 6 or 7 times before it straightened back out. Lifting off the throttle at the wrong moment would have put me into a sideways skid. (Had I been locked in 4high like I should have, it would have put me into a steady 4-wheel drift which is much more controllable) Since that day, I've never liked the programming of the automatic 4wd function. I would rather be in 2wd than in 4auto, and this mod did just that for me.
For me, at least, it's more of a control issue than a performance issue. Although, I have found 2wd low to be handy a couple of times too. Just last night I used it to idle the truck up onto ramps for an oil change. Worked like a charm with no worries about driveline bind-up when I backed it down and moved it over to park it on the gravel.
-Joe