I'm afraid it will harm your plants if you use it frequently - there's sodium in softened water, so using it very occasionally should be okay, but certainly not on a regular basis, and I wouldn't suggest doing it more often than two or three times per year max. Sodium builds up in the soil and causes plants to take up less water than they need, so ongoing, they'll suffer drought, even though you're watering.

More info here https://www.rhs.org.uk/advice/profile?PID=691

I'm no plumber but surely if you had an outside tap fitted, wouldn't that bypass your indoor supply?

First, water from a water softener should be routed and only dedicated to the washer, dishwasher, water heater, sinks and bath. A separate line for drinking water should be installed in the home that bypasses the water softener because one should never drink water from a water softener and the same should be done to an outside spigot for the same reason....too much consistent ingestion of sodium. Of course, drinking bottled water is always an option if you don't do so already.

Hope none of this applies to jug water filters? ie for drinking.

No Stera, it doesn't - the cartridge softeners do not contain sodium, they work with activated carbon impregnated with a little silver...

Whew!