[pfdietz]

> There is no disadvantage to being able to synthesise vitamin C,

Synthesizing vitamin C takes energy, energy that could be used for other biological processes. It's also possible excess vitamin C has some minor deleterious effect. For example, it's an antioxidant, and these render immune cells somewhat less effective against certain threats (which they use oxidizing chemicals to destroy). It's been found larger doses of the ACE vitamins causes increased growth of lung cancer, probably due to reduced immune attack.

Some have argued against this idea, though, although I'm not convinced by the argument (see if you can spot the problem.)

https://pmc.ncbi.nlm.nih.gov/articles/PMC3145266/