Trim, properly used, is a good thing. Proper technique in all planes that I'm aware of is to trim the airplane so that you could release the controls at anytime during climb, cruise, or descent and not have the airplane "wander off" on you. There is nothing more fatiguing than to have to hand fly a mistrimed or misrigged airplane for extended periods of time. (Autopilots don't like it much either.)
Like I've said before, you should probably avoid using the words "always" and "never" (or words like them) when it comes to aviation. For example, standard and acceptable technique when landing certain light taildraggers like the Aeronca Champ is to use full nose up trim on approach. But then again, you wouldn't have to worry much about "runaway trim" in a Champ and besides, where would it run to?
Pilots trim during the flare to relieve the back pressure that they are having to apply on the yoke. Remember from your flight training days that you trim for an airspeed. By adding nose up trim during the flare the pilots are, in essence, retrimming the airplane for a less than normal approach airspeed. As has been mentioned, in the event of a balked landing or go around, there's going to be a mad dash to get the airplane retrimmed and, in the mean time, the crew's going to need to put a lot of forward pressure on the yoke as the aircraft accelerates.
Is this a bad thing? It depends upon the characteristics of the airplane your happen to be flying and the conditions you're flying in. Solid IFR with an approach to minimums - probably not a good idea. VFR conditions, in an airplane where the stick forces aren't excessive, with landing assured and little or no chance of a go around - probably OK. But, just how difficult is it to leave the trim alone and add a few degrees of pitch?
Just a side note, if you're flying a jet and "working" the flare that much, the chances are you're going to be getting into an area of excessive float. A little excessive airspeed in the flare coupled with "working" the yoke to get a "greaser" and many airplanes have a tendency to get into ground effect and float and float and float. After that happens, the chance for a good touch down get really slim. In most jets, good technique is for your touchdown speed to be no less than Vref -10 knots.
'Sled