Why do some people own multiple drones? You only use the best drone you own all the time right? So what use case do you have for owning multiple drones?
Why do people own multiple cars, multiple televisions, multiple homes? Do you own multiple pairs of footwear? If so, wouldn't you always wear the most comfortable pair? Makes sense right? Except it doesn't work that way. We wear different shoes based on the purpose for that particular time. Leather shoes for a formal affair or work, sneakers for jogging, sandals for the beach, slippers for home, etc...
So drones are the same way. Each one is different and has its own best use case. Yes I have multiple drones but no I do not always use the same one. Even the definition of "best one" is nebulous. Some would say that the
Inspire 2 is the best of the lot. However it usually gets the least usage because it's just so big and heavy to carry around. For the best low-light pictures definitely the
Inspire 2 or
Phantom 4 Pro. Wonderful night shots. However the
mavic 2 Zoom with Hyperlight does a pretty good job.
For getting zoom capability only the
M2Z and
Inspire 2 will do.
For portability the Spark and Mavic Air are best. I just took the Mavic Air out last weekend because of the size. I can easily grab it and carry it over my shoulder while i walk to the local park. Walking any appreciable distance with the
Inspire 2 or even the
Phantom 4 Pro just isn't easy.
For windy conditions, the smaller lighter drones can't compete with the stability of the Phantom.
For commercial work, I need the best drone for that day's job. Usually it is the
Phantom 4 Pro but sometimes it is the
Mavic 2 Zoom and other times it is the
Inspire 2. They each have their specific qualities for the task at hand.
Then there definitely is the consideration of having a backup in case your primary unit is down for any reason.
Someone also mentioned familiarity with all the models. I give lessons and if I am not familiar with a specific model then I can't tailor the lesson for that person's specific drone. With DJI it's not a big deal since they all have very similar features, but it is still nice to know the specifics. For instance pairing a remote with a drone is so different on a Phantom 3 Standard than it is on a Mavic Air.
All that to say that yes there are MANY reasons to own more than one. It just depends on your specific needs and what you use them for.