Smell alone should not be enough to authorize the authorities to enter a home.
Agreed, but only because a home is a permanent structure.
However, if this was a search of a vehicle, it would be legitimate.
Although the judge's decision sounds absurd at first glance, I support it, even though I bet most cops can tell what marijuana smells like. The problem with allowing "smell searches" is that, smell being subjective (and often short-lived), a cop could search any house he wanted merely by saying he thought he smelled marijuana, and there would be no way to refute it either at that time or in court.
Once a cop has legally entered on probable cause, he would be free to note any other illegal objects or acts; the absence of found marijuana (which "might have been fully consumed or flushed down the toilet"), would not hinder prosecution on other charges. So if the judge decided otherwise, the 4th Amendment would become virtually meaningless.