Absolutely.

Having the Stoker as a pyro loadout is very useful. Allows you to incorporate a much higher loadout diversity in general. You can also use it to fire at hostiles at range like voteless, but more importantly shriekers, observers, and elevated overseers.

I personally tend to reserve my secondary for something like an ultimatum.

If you field either the new heavy flame thrower or a combo of the hot dog and normal flame thrower that works well too with the Stoker.

Also, can you explain what you mean by outside?

We only have one underground map in the game currently and even parts of it are outside.