"SteveB" wrote:

I believe that the hose bibs on the outside of my house are
hooked to the soft water system. �

I seriously doubt your outdoor hose bibs are connected to your
softened water, easy enough to check. But since water softeners
operate by on-demand it would use too much salt and place too much
stress on the unit were it used for for heavy watering as is usually
the case with outdoor water use. It's possible your hose bib is
connected to softened water but would be exceedingly rare. My house
has three hose bibs, none are connected to my softened water but the
one by the garage is tempered water, it is part hot water so salt can
be washed off vehicles during winter without it freezing.

Is there any disadvantage to watering with soft water?

There's no disadvantage but neither is there any advantage.