Is it possible to flash a custom ROM without a custom recovery image For an un-rooted device, almost all articles/forum posts I've read say that in order to flash a custom ROM you first need to flash (or boot) a custom recovery image like TWRP. 

I'm not sure I understand why. If the bootloader is unlocked shouldn't it be possible to manually flash images that make up the custom ROM to the required partitions with fastboot flash &lt;partition&gt; [ &lt;xxxx.img&gt; ], and leave the stock recovery untouched.

I realize that this may be difficult and that there is no benefit in doing it this way, but I'm curious to know whether or not it is possible, and if not, why?
Yes it is possible to flash images via Fastboot ? 

With an unlocked bootloader you should be able to flash some partitions without overwriting the stock recovery, however each system usually requires a unique recovery to handle factory resets etc..

I have had issue's in the past with Maximum flash size, it wouldn't let me use Fastboot because my system.IMG file was too large to send via Fastboot.. That then required splitting the image into smaller files and flashing the smaller files.  

You don't specifically need a custom recovery, however it makes everything a lot easier... For example, most custom firmware requires you to do a full backup of your device prior to installing, you need a custom recovery or rooted device to be able to make a full backup... 

Why do they say " Use a custom recovery " ... Well this is probably determined by the firmware creator, they've most likely built everything into a single Flashable Zip file, using Fastboot won't work because the zip file contains more than just .IMG files, usually a ZIP firmware will have a folder called system, the system folder is copied to your device by the recovery, not flashed  .. each firmware usually contains 3 parts ( boot.IMG, System folder, additional files)... However some firmware's contain a complete system.IMG file instead of a folder.. These files can be pulled from the firmware and flashed via Fastboot, however Fastboot can not flash folders ( only a custom recovery can do that )  

... I'm starting to get lost here...