What was the intended meaning of the quotation marks around "sinners" in the old NIV? Take for example Matthew 9:10-13:


  While Jesus was having dinner at Matthew's house, many tax collectors and "sinners" came and ate with him and his disciples.
  
  When the Pharisees saw this, they asked his disciples, "Why does your teacher eat with tax collectors and 'sinners'?"
  
  On hearing this, Jesus said, "It is not the healthy who need a doctor, but the sick. But go and learn what this means: 'I desire mercy, not sacrifice.' For I have not come to call the righteous, but sinners."


My younger self thought of them as similar to sarcasm quotes, to be read while smugly preening over the blindness of the Pharisees. That seems entirely inappropriate for a Bible translation, though, and the only explanation I can come up with for them being there in the first place is to imply that the Pharisees did not consider themselves sinful, or to imply that Christ's dinner guests were not really sinful after all. And then that old saw that goes "well, we're all sinners" comes tramping into mind...

I notice that the NIV 2011 took them out, but have the translators (or anyone else) ever explained why they were there in the first place?
This is not addressed in the Preface to the 1984 NIV, so we must look elsewhere for clues.

The best explanation I've found comes from Craig S. Keener's commentary on Matthew:


  Some take sinners here to mean the 'am hā'āres common people whom the Pharisees despised for their lack of adherence to Pharisaic food laws (as in Jeremias 1972:132; thus the quotation marks in the NIV); more scholars today lean toward the view that it means sinners in a more blatant sense. (source)


So the NIV translators make a judgment call – when the word sinners is used to mean something more like "despised by Pharisees" than "those who violate God's law," they put the word in quotes.  

James A. Brooks, in The New American Commentary, explains the matter similarly, regarding Mark 2:15:


  The NIV is quite correct to put the word "sinners" in quotation marks to indicate that it is being used with an unusual meaning.  The reference is not to immoral or irreligious person but to those who because of the necessity of spending all their time earning a bare subsistence were not able to keep the law, especially the oral law, as the scribes thought they should.  As a result the scribes despised them.  Perhaps a better translation would be "outcasts" (GNB). (source)


But, as Keener notes, this is an interpretive decision and there's some disagreement over it.  Thus it shouldn't be surprising that other translations, including the NIV 2011, don't adopt this stylistic approach.