The 28 is the Nimbus on a diet. It keeps the FF BLAST PLUS midsole and the PureGEL under the heel that make this ASICS' softest road shoe, but it weighs 281 g against the 27's 305 - 24 g less shoe for the same landing.
It suits runners who wanted the Nimbus ride and found the 27 a lot to drag around: easy days, long runs on pavement, and anyone spending hours at a conversational pace.
It is still a max-cushion trainer, not a light one. At 281 g it is heavier than most daily trainers and it goes vague the moment you ask it for pace. Buy it for the easy end of your week; if you already own a 27 and get on with it, 24 g is not a reason to replace a working shoe.