Why did the Pharisees need Judas to betray Jesus? I quite don't understand this whole thing. Jesus and his disciples are hanging around the city all the time. Why can't the Pharisees arrest him at their leisure? Why do they need Judas to tell the soldiers which one Jesus is?
It's something you probably don't think about much, but we have reason to believe that Jesus really wasn't all that distinctive in his appearance.


  John 8:58-59
  
  58 Jesus said unto them, Verily, verily, I say unto you, Before Abraham was, I am.
  
  59 Then took they up stones to cast at him: but Jesus hid himself, and went out of the temple, going through the midst of them, and so passed by.
  
  Luke 4:28-30
  
  28 And all they in the synagogue, when they heard these things, were
  filled with wrath,
  
  29 And rose up, and thrust him out of the city, and led him unto the
  brow of the hill whereon their city was built, that they might cast
  him down headlong.
  
  30 But he passing through the midst of them went his way


This is the act of someone who is very ordinary-looking and able to blend in with a crowd at will.  See also Isaiah 53:2.  The context makes it clear that he's prophesying about the Messiah here, and he clearly states that he does not look like what you'd expect of a powerful, charismatic leader:


  ...he hath no form nor comeliness; and when we shall see him, there is no beauty that we should desire him.


As Dancek pointed out, Jesus reminded them that they could have come for him any time he was in the temple teaching.  It's not hard to find him then: look for the one who's the center of attention!  But looking for him at night, when you don't really know what he looks like and even if you did, that description might match any of thousands of Jews? (And this was Passover season, so there were a lot of extra Jews in Jerusalem at the moment.)  The only way to make sure you've got the right guy in a situation like that is to get someone who personally knows your target to identify him.  And so that's exactly what they did.