I've never used NetWare 5.x 6.x but if I remember correctly versions 3 and 4 required you to boot to DOS partition and then load NetWare server files from it. I could never understand the logic behind it. Why wouldn't NetWare just boot into its own partition and load the necessary server files from it?
In order to load files from the SYS volume NetWare needs to have a device driver for the kind of disk your SYS volume lives on. Considering that you might put your SYS volume on an MFM drive, or RLL, or ESDI, or SCSI, or IDE, or any of a variety of different RAID controllers, we are talking about a lot of different devices your bootloader needs to be able to handle.
Being able to load the device driver from a DOS partition simplified things quite a bit. While none of the NetWare servers in my care were ever configured to do so, I suppose you could theoretically boot from a floppy containing server.exe and your disk device driver so that 100% of your hard disk capacity could be dedicated to NetWare volumes.
On top of the driver issue, NetWare supported large volume sizes (larger than your system BIOS might support). Booting from DOS avoided the need to be careful to place your SYS volume within the first 504 MB of the first physical drive (or whatever your particular BIOS limitations were). Instead you just had to create a small DOS partition somewhere your BIOS could boot from, and you were then free to put your SYS volume anywhere else you liked, and make it any size you like, even placing it somewhere your BIOS didn't support booting from.
From Network World:
NetWare never ran on top of DOS. A server was booted to DOS solely to run the NetWare boot loader (a DOS program). It didn't need to do that, it could have booted directly but that would have required Novell to build its own BIOS loader to initialize all of the hardware. The NetWare designers felt that there was no need to reinvent the wheel, so let DOS handle that.
Novell was not the only operating system vendor to use MS-DOS as a boot loader: Microsoft did much the same for 16- and 32-bit Windows, especially from Windows 95 onwards where returning to DOS was not a standard feature. (Windows NT and its successors did not use DOS as a loader.)
This paper from Drew Major et al. doesn't explain why they used DOS for booting but does offer insight into Novell's design philosophy:
The NetWare operating system is designed specifically to provide service to clients over a computer network. This design has resulted in a system that differs in several respects from more general-purpose operating systems.
I would imagine booting from DOS to be one of those differences that places convenience and simplicity over traditional OS goals.
If you're interested in learning more of the design of NetWare, this paper is a good place to start.
Finally, Novell did eventually write their own boot loader.