Sometimes I find that I want to refer to a specific period of G1 in a way where referring to G1 as a whole isn't precise enough, but referring to year is too precise. Or sometimes it just helps to remove ambiguity.

e.g. G1 Constructicons
L-R: 1985 Scrapper, 1992 Scrapper, 1993 Scrapper, Encore Scrapper

I sometimes see sales posts for 1992 Constructicons advertised as "G2," even though they are NOT G2 Constructicons. Colour variations aside, the Late G1 Constructicons cannot combine to form Devastator, so let's say I were trying to collect G2 Devastator and I decided to purchase G2 Scavenger. I see an online sale for what's advertised as "G2 Scavenger" and I go ahead and purchase it, only to discover that my Scavenger is missing the connecting piece that attaches it to Hook. And if the Hook I purchased was Late G1, then it would be missing the connecting piece to attach to Bonecrusher. Oops, I now cannot form Devastator! The toys aren't necessarily broken, that's just how they were made. Being so similar in colour and appearance, it's understandable how some people might confuse the Late G1 Constructicons with the G2 Constructicons, but there is a substantial difference between the two sets. Even if you were to purchase the Late G1 Constructicons MOSC as a complete set, they don't come with any of the component accessories for Devastator either - no forearms, no fists, no chest plate, no head.

Other examples: Early G1 Barrage (1985) is an Insecticon, but Middle G1 Barrage (1990) is the front half of a lunar truck. Early or Middle G1 Afterburner (1987) is a motorbike, but Late G1 Afterburner (1991) is a Ferrari F40 sports car. Although both G1 Sky Highs are Middle G1 according to each of these proposed models, as the first was the Pretender released in 1988 and the second was the Micromaster released in 1990, so you're right, there are times when even dividing among these periods won't be precise enough and referring to explicit years of release will be required. It all depends.

At other times, you're right, you don't need to divide it at all.
e.g. I want to see all of G1 eventually done as CHUG toys. And by that I do mean all of G1. Because when some people say that, they don't mean it. But, mate... I don't just want Early G1 stuff like Venom, Chop Shop and Hubcap... I also want every Middle G1 TF done as CHUGs like the Sparkabots and Triggercons! I want the rest of the Autobot Targetmasters done as CHUGs (we've only ever had Hot Rod and Scoop done (also Blurr and Kup if you count TR/LG)). I was stoked to get TR Quake and Siege Spinister! I also want all of Late G1 done as CHUGs too, including the Motorvators, Action Master Elites (maybe not a Titan Class Omega Spreem ) and Lightformers. I preordered the LGEX Big Powered set, jumped on TR Overlord (only to later replace it with LG Overlord), and if they ever redecoed CW Defensor and Bruticus as CHUG Guard City and Battle Gaia respectively - I'd buy them in a heartbeat! So yeah, when I say that I want all of G1 done as CHUGs, I literally mean all of G1. Bring on CHUG Time Warrior!

But there are other times when I feel the need for the division.
e.g. I'm mostly interested in Early G1 for MPs, and also Early G1 seems to be what sells for G1 MPs.
The only Middle G1 MP they've ever done was Star Sabre, and even then, it was the result of a fan's choice poll in 2013. Whereas Early G1 you can more or less release an MP toy and be guaranteed to sell it. Star Sabre is the only Middle G1 MP I own, and even then it was a freebie. :/ They've never made an MP toy of any Late G1 characters, and even I doubt that I'd buy any of them. I love Deftwing, but I wouldn't buy a Masterpiece Deftwing.
"But they made an MP of Targetmaster Hot Rod, and he's Middle G1!"
Well that depends on what you define as "Middle G1," doesn't it? (and thus the purpose of this poll!) If you go by the Mathematical Model, then yes, he's Middle G1. But by the Popularity Model he's Early G1, and by the Age of Distinction Model he's an Early G1 Silver Age character.

And that's the other thing, if you were to say, "I'd buy any of the Early G1 Autobot Cars if they were made as MPs"
By the Mathematical Model this includes all of the Diaclone Cars from the first two years but also includes Hot Rod, Kup and Blurr. As dirge alluded to, they're a different flavour from the Diaclone Cars because they're futuristic/fantasy vehicles rather than being based on licensed real life vehicles. Similarly this model includes Scourge and Cyclonus as part of the Decepticon Jets.
The Popularity Model is the same as above but also includes 1987. So now suddenly your Autobot Cars include the likes of Targetmaster Hot Rod (which has been done), but also Targetmaster Blurr, Pointblank, Sureshot etc., and I'm not sure if people who'd happily buy MP Trailbreaker, Hoist, Mirage, Jazz etc. would be equally enthused to buy MP Targetmaster Kup, Crosshairs etc.
By the Age of Distinction Model you could say, "I only want to collect all of the Early G1 Golden Age Autobot Cars as MPs." This stops at 1985 so it means that you're only interested in the "Diaclone Cars." You want MP Jazz, Trailbreaker, Hoist, Mirage etc., but you wouldn't go so far as to want an MP Kup or Pointblank. You've purchased MP Starscream, Thundercracker, Skywarp and the Coneheads, but you may not want an MP Cyclonus and Scourge.

It all depends - we all have different criteria and the purpose of this poll is to create a general consensus.