Personally, i wouldn't be without one - in fact i never ride without it, even solo. So no cons from me, apart from the orice - but i would replace it with another hondaline one if i ever needed to.
Pros:
High level brake and tail light.
Space to put helmet and gear when looking around places. We stow our textile suits, boots and helmets for en-route stop offs. I cant bear walking around in heavy, sweaty gear.
Somehwere secure and lockable rather than having gear tied on loose
Easier to access than panniers. We use liner bags in the panniers to make it easy to pack the luggage way from the bike ( in tent, or in b&b) and to keep stuff together when the side lid is opened. Individual items go in the top box and can be kept locked in there overnight.
Top box is easier to keep things dry when opened in the rain.
Top box is easier to access then left pannier when on side stand.
I dont have to worry about my pillion if i need to accelerate rapidly.
A handy place to stow and charge an ipad or notebook on the move.
Prefer to ride with top box than a tank bag
But the choice is a personal thing. Our packing has evolved over the last 16 years of touring - both camping and b&b nearly always 2 up.
Whatever you decide, it is well worth having an overnight dummy run, in the rain, to see if the arrangement works before your long trip.
Good luck with the trip and the planning.