This is in danger of getting deeply pedantic, but the intention (bearing in mind this wasn't my idea...) was a quick reference for people who know very little about 300s, primarily based on exterior appearance. I agree with what you're saying, and don't debate for a second that there were significant changes between 78 and 79, but the cars do look very similar from the outside, whereas the change in 1982 was much more significant.
I'm not advocating that either group change their way of referring to the cars...although there is a slight danger that people could get confused, as our respective mk2s and mk3s are different cars. On that basis, it's probably best to just talk groups of model years. (i.e. mk2/mk3 referred to as 82-85, etc). The other factor is that I can't remember the last time I even heard of a 76-78 car in the UK. The 79-81 cars are rare enough, but the really early ones scarcely exist at all.
I don't think the 850/V70 comparison is a fair one. There were significant exterior changes between the two, including quite a bit of sheet metal (wings, bonnet and bumpers, plus extensive changes to the saloons including the rear screen and bootlid). It's closer to the 81/82 update, if anything. Externally, the only 'obvious' changes (off the top of my head) in 78/79 were the number plate relocation and addition of side repeaters.
Anyway, pedantry aside, the main thing is that we know which cars we're talking about...which I think we do, especially if we clarify with model years. All good fun
cheers
James