I'm not against the idea, but I wouldn't want one for myself.
As has been pointed out, the Joycons will have to remain attachable to the outside of the clamshell.
Both DS Lite and 3DS had widespread problems relating to the clamshell. The DS Lite had very fragile hinges, even a small drop off a coffee table could cause it to break. While the 3DS had improved hinges, they still had problems - wear and tear over time causes the hinges to loosen. The big issue with 3DS was the bottom screen biting into the top screen. So Nintendo really would have to make sure they get the design right with the Switch because Switches are much more expensive devices.
But the question from a company standpoint: What business and creative goals do a clamshell Switch contribute to?
That form factor would allow for a much larger battery, the problem with the Joycons is from a hardware POV they're basically "free loading" off the main Switch while not really contributing anything, even battery power. So that means the main unit of the Switch has to contain all the hardware including a large battery in limited space.
So you're kinda stuck with a small battery. With fold out design though you don't need Joycons attached to the system, the controls can be integrated on the main unit making it less wide, but it could actually still be wider than the base Switch unit, that space could then be occupied by a larger battery.
I think the other thing is VR might be a big feature for Nintendo next-gen, Labo VR I think was the experiment working towards it, it could really put a fresh spin on a lot of IP like Mario Kart (which already is being tested as a VR experience in arcades).
The problem with the Switch being slid into a wearable headset is that it makes the display very top heavy, especially if the user ever moves their head slightly forward or down. You need a counter weight at the back which makes the whole headset even heavier. And that might be workable for adults, but for younger kids ... probably too heavy. In a fold out design it's possible the screen itself could just detach and that portion could slide into a VR head and be very, very light, ideal for VR/AR experiences that kids want to enjoy too.
I think they'll make a VR headset (official) next gen and probably have some neat Nintendo touches like much easier to use/less intimidating, lighter, few or no wires, and fun little touches like a blow motor that simulates air hitting the face (very Nintendo-ey). The problem with that is how you get the display. If it has its own display then it has to be pretty expensive like the PSVR is. If it has no display then it can be cheap (just a plastic mask) but the weight of the Switch becomes an issue if you're sliding the entire system in there especially for kids.