Except for in scripts. Debian guarantee that the output format of apt-get will never change and thus it's safe to use in scripts that parse the output, whereas they don't have the same guarantee for apt, which can change between releases.
Aptitude is great (my favorite way of managing packages), but it's a TUI program. You can use it as CLI, at which point it mimics apt-get.
So I would say it never attempted to unify apt commands, by rather it successfully provided a user friendly way to do most (all?) of what you could do with apt CLI tools.