Because the "a.b" is not a key, but a table "a" that contains an entry with key "b". If you write "a.b = 1" then that integer is not a table, and can't have an entry called "c".

Because it avoids these problems, your "name.alternative.first" example seems perfectly sensible to me. Alternatively, the name might be an array:

    [[name]]
    first = "Bob"
    last = "Smith

    [[name]]
    first = "Robert"

It's probably best to think of TOML as convenient syntax for creating a JSON-like structure. What kind of JSON would you expect as the result of your config examples?