@T**
I too know people who do that BUT they are locked into use an agency year after year as the immigration officer only has to look at the stamp to know it's a back-door, under the table visa extension stamp.
That people do it doesn't mean it's the correct way to go about it IF there are other alternatives. Believe me IF there was ever any blow back from it, the person whose passport has the under the table stamps in it would bear ALL the burden not the agent or agency who got the stamp for them..