Extensions add "support for USB 2.0 and USB 3.0 devices, VirtualBox RDP, disk encryption, NVMe and PXE boot for Intel cards."

I'd argue the added features themselves are mostly shit. If you need hypervisor RDP or FDE there are much better solutions than VirtualBox.

I don't care much about other features either, but accessing USB devices is often invaluable.

Often? For what kind of device? Input devices don't need to be fast, and storage is usually better off with a shared folder anyway.

There’s lots of other hardware that people use. For example, the usb cable to connect to my BMW needs usb 2.0, and the software only works under Windows XP, so I have a virtual machine for that.

For tools like Vagrant, not at all. They are very useful if you want to attach USB devices directly to the VM, use the built-in webcam and so on, but I've used VirtualBox for years without installing the extension once.

USB 2.0 and 3.0 are not fully supported without the extension pack.

Which again are irrelevant for the common “just for Vagrant” use case.

I have used it for testing on different Windows browsers, to check whether https://github.com/rbanffy/3270font would install correctly, and running some Windows software from my bank. The only time I needed the extensions was when I needed to reflash my Blackberry 10 DevAlpha (whose battery, sadly, is dead - I loved that phone)

How so ? (don't confuse the extension pack with the guest addons though)