Dual booting is common for people playing games. While steamplay/proton works well enough for some games a lot need either a lot of hacking or a full blown windows to run. You can do GPU passthrough VMs if you have two GPUs but I have a miniITX board and no iGPU so that is not an option for me.
Personally when dual-booting I don't share anything at all between the OS'es, but my understanding is that most do.
I haven't dual-booted in ages, but sometimes use NTFS for USB drives or inspecting drive images from Windows machines / VM guests.
I used to do this in my college days. Had some work (Unity projects especially) that would run into FAT32 file size limits and NTFS was the one more advanced filesystem that at least somewhat worked on all three OSes, albeit read only on OS X. exFAT support for Linux didn't exist at the time and FAT32, exFAT and NTFS are still the only three OS portable filesystems. Windows cannot handle HFS+/APFS, ext* is not supported on Mac and poorly supported by third party software on Windows, and ZFS/btrfs/XFS/UFS/etc are out of the question for both Windows and Mac.
Amusingly, there was actually a historical Mac ZFS effort that was, I think, sidelined in favor of APFL: https://arstechnica.com/gadgets/2016/06/zfs-the-other-new-ap...
They had a GPT GUID for it: 6a898cc3-1dd2-11b2-99a6-080020736631.
Time Machine looks like it was designed for ZFS and some poor engineer at Apple had to graft it on top of HFS.
I really wonder how common that need is and whether it's worth adding this sort of baggage to Linux.
Honestly, for me, mounting Amiga file systems would be more useful.
ntfs-3g is adequate for me, so I don't care one way or the other if _this_ driver gets merged. And Linux is not a commercial OS, there's not some prioritised backlog controlling everything goes in, a lot of the time its what someone is willing to make the case for and sonmeone is willing to put in the work for inclusion/support. See all the esoteric/obselete cpu architectures that are still supported in the kernel - there are absolutely more users mounting NTFS drives than running Linux on PPC for example.
However, "mounting windows filesystems" is undeniably a more common case for Linux users as a whole than "mounting amiga filesystems". Maybe WSL changed this, but Windows/Linux dualbooting used to be one of the two gateways for Linux users alongside VMs. Perhaps the absence of a apfs-3g is a contributor we see less Mac/Linux dual boots.
On Linux, my most common scenario of deploying Windows is in small VMs and, when needed, mounting host folders from them. It's true these VMs will probably not run some PC software well, but I have a Windows PC for that. The other way around we have WSL and WSL2 on Windows, again kind of erasing the need to dual boot from the Windows user side.
The money for them here is in support. Maybe they have licensees that bind them to support contracts and merging software into mainline Linux counts as a "best effort" to make others chip in at keeping it functional as the kernel evolves.