Regarding the value of "partial protection", the TAS belts would seem to offer just that: protection from weather only. They seem to be the ultimate in that, really: they're likely to deflect anything from dust storms to heavy rain to poison clouds to scorching sun. Plus they serve as spacesuits in benign environments! No doubt this is something the user is willing to pay for, the price apparently only being the weight of the belt (or the even more compact buckle thing we see in TMP - easily thrown away if the power source starts to misbehave, BTW!) plus the annoying glow that reduces combat value by preempting all attempts at stealth.
It's difficult to see why such a thing would be ditched after TMP. Although arguably all the necessary hardware could have been stored in one of those big pockets of the later TOS movie parkas, or even sewn in.
In combat terms, though, there seems to be something horridly wrong with future technologies or materials, and not just in terms of "full protection". The extensive and cumbersome Klingon armor does nothing to stop even the feeblest "partial threats"; the Cardassian whole-body armor sucks as well, its darkest hour probably being "Indiscretion" where it fails to protect Dukat's bottom from a sharp stone!
In contrast, the innocuous-looking coveralls worn by Starfleet ground troopers seem to hide a trick or two, as we see from the layered charring on Lieutenant Burke in "Nor the Battle". He nearly survived a disruptor blast right in the chest! I'd be willing to believe those coveralls have all sorts of "partial protection" abilities as well, being good against cold, heat, rain, knives etc. (since in contrast with the Klingon and Cardassian costumes, the coveralls have not yet demonstrated shortcomings in an episode or a movie). Alas, there's no associated headgear.