It is your charter. It is your crew, it is yours to decide.

Having a second dinghy means more flexibility in planning and decision making. With a large party, it means choices in where to go, when to go, what to do, and not everyone has to go and do the same thing or be " stuck" on the boat at a mooring. Or all need to be ready at the same time for the dinner run. Or whether 2 or three can go do re-provisioning/shopping/sightseeing while others go elsewhere. Just HAVING the flexibility and options makes a big difference with a large party. Your decision, your experience. Over 9 people, that 35 bucks a day is less than 4 dollars per person per day.

Your charter, your party, your vacation. It's kinda like extra towels and sheets and pillowcases and TP. Not totally necessary, but nice to have.