I'll do a bigger write-up of why the purported logic is terrible (that you can't 'see' things behind you and it's an optional rule in the DMG), but for everyone using this logic please be aware that using the former argument will absolutely break hundreds of spells and class abilities.
Many, MANY spells and abilities use language that requires you to be able to 'see' your target. So, take the below example:
W ----- X ----- Y ---------| ---------| ---------Z
In this scenario, let's say X wants to cast Bless on W, Y, and Z. Using the logic of Facing, this is impossible. They cannot 'see' each of the targets all at the same time. Add in another target on the remaining axis (Upcast Bless at second level) and it just shows how impossible that will be.
Additionally, you now need to introduce the other rules for the Facing rule. A character with a Shield equipped only gains that benefit against attacks coming from about 1/3 of the area near them, the other 2/3s is useless. Barbarian Danger Sense now stops working a lot of the time because you can't 'see' the danger. You have to start modifying HUNDREDS of spells and abilities because of one dumb rule that I personally have never heard of being used by any DM, ever.
Or just change it to be a +2 modifier so it is significantly less impactful on gameplay, or ideally, remove backstab and instead use the Flanking rule with the +2 modifier to increase the amount of tactical maneuvering required to gain a benefit in melee combat, while having no need to rebalance the hundreds of other spells/abilities.
