That's the 64000 dollar question. Should you? I vote speak softly but carry the big stick.
The only necessity is to be able to keep up with your drummer. Then a vocal PA has to keep up with you, or you have to dial it back.
It sounds like the main use you would get from a second cab is outdoors.
A vertical stack is very nice for monitoring too.
Mind out you don't inspire an arms race with your skinny stringers.