I am a professional photographer who shoots almost exclusively with Nikon. In the rare cases, I use a different brand like Canon or Olympus, or technically a DJI drone. I haven't yet shot with the Sony A7 series, nor do I plan to.
Over the course of the last few days, there has been so much consternation over the lack of dual memory card slots. I just wanted to throw my opinion out there.
First, many of these people that are upset the camera doesn't have two slots seem to be the same people who claimed they would never switch to mirrorless in the first place because it's not professional grade. If that is the case, why are they complaining? They wouldn't use it anyway. They just seem to be complaining because they have a keyboard and an Internet connection and have nothing better to do. I also would be willing to bet this seems to be a non-issue with their drone....hmm.
One must realize as a business, your goal is to be profitable. If you aren't, then you're not in business anymore. Nikon took a giant leap, and a good one at that. I feel they exceeded my expectations in almost every way. One of the things you don't want to do is directly compete with yourself or dismiss your own product lines. Unless it's a clear upgrade, it's just bad business sense to do this. Nikon has a few main product lines, everything from your kit-type consumer cameras, to prosumer and professional. Nikon just released the D850. For most things, it's the perfect camera for general purpose. It wasn't designed to compete or replace the D5, if it did, then there would be no reason to sell the top line of bodies. This is exactly what the Z series is about. If you have a new camera that, essentially, outdates your newly released D850 and directly competes with your D5, you've created a problem business-wise. Furthermore, adding all the new features, such as the new Z mount is a huge [necessary] risk. Dumping more R&D money into a product that is not only new, but new to Nikon as well. Cautious, yet aggressive.
As far as the memory slots are concerned. I am a huge proponent of dual card slots just for the backup mode. I love having the simultaneous writes for a few reasons. Redundancy, peace of mind and flexibility. When I have finished photographing a certain area, I like to take the SD card from my camera and put it in my wallet. So I have the data in two places, and one is physically on me, until I can copy them off to the computer and uploaded to my server.
For paid gigs, weddings, remote hotel/property shoots, it's almost a must. But then you must ask yourself, why would you be doing it with a new, untested product anyway?
That said, I am a little disappointed it doesn't have two slots. However, I also recognize some reason as to why. Some may be pure speculation; cost, feature competition, market testing, etc. But what I have said previously, this is not a professional grade camera [yet] and therefore, it's not a deal breaker. I have my D850 and D810 and they have done the job I needed and will continue to do so. If I were to get the Z7, it would be for backup purposes or a camera to take when I want to hike, sightsee, or something of that 'nature'.
This is only the start of the full-frame mirrorless cameras from Nikon and I expect them to perfect it over time. Remember, Sony has many iterations of the A7, and they still don't have it right. Nikon did a lot of things right the first time, even doing things Sony hasn't done in the last 4-5 years.
In summary, I prefer dual card slots. But it's not the end of the world. Most real professionals wouldn't dump their current equipment for the newest shiny object anyway. And it seems like the biggest complainers can't distinguish between the different lines. My guess is their photography consists of shoot, upload, filter, hashtag.
I will update this as I feel like adding more.
I welcome any comments of course, but remember these are my opinions and I reserve the right to disagree with you as well.