In astronomical contexts there are good alternatives to the epoch (like TDB, mentioned in another comment). I interpreted the top-level question as relating to day-to-day measurements and representations of time, presumably for use by extraterrestrial humans, since the commenter asked in the context of handling time zones and units like weeks and months which are irrelevant in a scientific context.

UTC is not a great choice for a universal interplanetary time system, as it's designed to work on just one planet, for just one calendar system. It'll work fine while we're asking humans on Earth to oversee robots on the Mars, but will fall down pretty quickly once humans take permanent residence on celestial bodies that don't have Earth's rhythms.

Well, the general idea seems usable enough; X time units from a specific reference point. Conversion is then done to whatever local time-keeping system you might have.

Leap seconds are based on conditions here on Earth and wouldn't really be needed on Mars or Venus. They might have different peculiarities though, and they wouldn't be usable on Earth. In that situation, a single point of reference would be useful for conversions.

Right; "X time units from a specific reference point" is what TAI is. Taking Unix time and removing the notion of leap seconds is effectively a circumlocution for exactly that standard :)