The reality of the situation is; without Xander, Buffy would be dead. He saved her life in 'Prophecy Girl', and saved the world in 'Grave.' I sort of see Xander as the quiet hero. He makes a difference but doesn't brag about it. Remember in 'The Zeppo', when he bravely prevents that bomb from going off, saving the entire student body? He never even tells Buffy or Willow about that. That was real. I admire him for that.
Sure, Xander has made some bad calls - for instance, I never agreed with what he did regarding Angel, where he purposely stalled in relaying the information to Buffy about the possible restoration of Angel's soul. If he had got there in time, it might have been possible for Buffy to not have had to kill Angel. It might have not been necessary. Xander hated Angel, was insanely jealous, and his action in that instance proved how we was willing to do anything to get rid of Angel.
Regardless, Buffy trusts him with her life. He is her "strength." I truly believe that Xander is her best friend and always will be.