A lot of real-world data is already in base-10 for obvious reasons, and so an arrangement that lets you add, subtract and multiply those without worrying is worthwhile, even if it can't handle something more exotic.
Maybe we should also add data types to every language that can convert exactly between inches, feet, miles and every other non-base-10 unit?
The argument "we want to look at base-10 in the end so it should be the internal representation" is really weak and ignores basically every other practical aspect.
