Got to be 4 for me aswell. To say i wouldn't be happy if i saw my girlfriend holding another guys hand would most likely be an understatement and thus i would never do the same to her.
As said early on in the thread, holding hands is most definatly a sign that you are an "item" (don't like the saying, but you get the gist of it) and that you are together.
Also, another point, if you were helping someone because they were drunk and couldn't walk properly, holding hands wouldn't cut it and at that stage you are linking arms or putting your arm around the other person for support.
Guess what i'm trying to say there is that holding hands TO ME atleast defineratly says that 2 people are together, where as other things such as linking arms and putting your arm around someone for support while "friendly" doesn't hold quite the same context
how-ever, i think a lot comes down to the circumstances in which everything happens.