there's no denying a properly trained, properly handled dog is reliable.  there is reason to doubt this particular dog meets those standards.

beyond that, having adequately trained and handled dogs investigate shipments or baggage is considerably less potentially problematic. should a suitcase or container cause the dog to alert on residual scent molecules (an indicator it may have previously held drugs), agents can open and inspect them.  if contraband is found, the consignor/consignee or passenger can be arrested and charged.  if nothing's found, no harm, no foul.  (if youve flown anywhere since 911, your bags may have set off the ion detector used to sniff out explosives--not an uncommon occurence).  the handler is presumably checking things at random outside the presence of a person who might otherwise match the handlers subjective profile. 

on the street, a person's or vehicle appearance may factor into the procedure because the handler--unlike the dog--is not going to be unbiased.   the dog cant testify whether its actions were 'cued'--perhaps even unconsciously--by the handler.  nor can the dog explain, hey i smelled methamphetamine residue altho some other drug it isnt trained to locate is subsequently found. 

i hope he demonstrates the muzzle for the jury