I too occasionally do this. For the exact reason Foxhound mentions, and the secondary reason Four Dogs mentions.
I don't do it often.
Take, for instance, the situation where maybe midway thru a MTT and you have a bully at the table, and no one has seen any hand of his for a long time. That would be a time to see just what the **** he has been betting.
At which point it was a tactic in my whole game strategy.....to understand, to the best of my ability, each of my villains. Unless he was showing me something extremely strong at that point, I would have just done serious damage to him, without taking all his chips, which is where the problem arises about softplay. But more than likely he would have had me covered in that hand, and I feel I would be justified in my action, and could probably convince any judge of my actions that I was OK in doing it that way. Not positive tho.
Now if it were a standard thing I was doing like every hand I was involved with, rather that suffer the wrath of rule, I would probably become the runt of ruin from all the villains at my table.
Overall, I think value betting, however small, when you get to that point, is the better path.