Part of it is to have a 4 door to use when family is visiting/picking up friends/coworkers from the airport or whatever. I think you rocked the C6 as an only car for a while, so you have some idea how handy extra seats can come in sometimes.
It's like the SUV/truck deal. Most days out of the year it's just one person in the vehicle, but they have it for the occasional boat towing/cargo/whatever.
C6 was my only car for about 3 years, I bought the 4Runner when I bought a house plus had a large dog (and later on a second one). That said, I also comfortably paid cash for both and the 4Runner in particular was a pretty small % of both my income and my liquid savings at the time. I'd say unless you're in a position to comfortably pay cash for this second car, don't even consider it (I don't think you said you're financing it I'm just sssuming that's a possibility given that's what most people do).
For things like friends getting a ride from the airport ... dude, Uber. Things like uber have made those occasional needs much much easier. Have your family rent a car when in town, even offer to pay for it. Don't pay interest, insurance, maintenance, and eat depreciation all year round for something you need for 2 months and then only occasionally thereafter.
Another advantage of the monthly rental is it would give you an idea of just how often you use those additional seats. If you find it's something you're unable to give up after the 2 months, then you could reconsider (or replace the Miata).
Not trying to rain on your parade, or turn another "what car should I buy?" thread into a carspin money advice thread. Just have to objectively consider, is the utility you'd get out of it really worth the costs?