Sorry to resurrect such an old thread – but I wanted to share a few thoughts
FOR
1. It’s surely not the worst Averah in the world, so one should be very careful before publicly rebuking people for it.
AGAINST
2. It’s interesting to note that many of the people who do this are not exactly Oisek BeOrayso Todira. Suddenly during CH they get an intense urge to open a Sefer, yet the minute Davening finishes they’re out of the door. It would really seem to be Atzas Yetzer Hora.
3. Ironically, according to MB quoted by OP, the more rabbinical the person is the more problematic it is, as others will learn and copy.
4. Kaf Hachaim quotes a Sefer that this is an example of someone whose Torah does not produce Nachas ruach to Hashem.