Multi class fighter/thief vs thief

Is there much of a downside to being a fighter/theif compared to a straight thief? Seems like you get plenty of theif points without needing to be a straight Thief and you'll get a bit more combat ability. I always felt that there should be only 1 way to combine dual classes and they should have either gone with dual class or multi class not have both options but that's kind of off topic so to be clear I'm looking for feedback on multi vs single not dual vs single.