These 'leaks' permit chemicals to attack the unprotected surface as well as the bonding interface. Each coat is a contrasting color to aid in spotting thin or missing spots. Older epoxy formulations were generally more brittle than many of their modern counterparts. Different coatings handle abrasion and wear better than others. These fibers, in addition to improving wear resistance also seem to work like rebar does in concrete, localizing any damage to the coating.
Chemical resistance varies between different coatings.
Strong acids, for example, will quickly destroy many coatings. The slabs will move and the coating over the joint will crack and probably disbonding big chunks of coating in the process. Don't paint over expansion joints. There are other methods for dealing with them. Salts were already covered above.