How often do you actually want the nth character as opposed to the nth grapheme?
There is pretty much no case where indexing by character actually makes sense because it is almost always incorrect and it is always inefficient.
Indexing by byte is rarely useful, but it does have some usefulness since it can be used correctly and efficiently since you can easily find the next or previous character by searching a maximum of four bytes for the a byte that has a MSB of 0. If you want to do something like get a &str that would fit in a n-byte buffer, then byte indices will let you do that efficiently and correctly.
There is pretty much no case where indexing by character actually makes sense because it is almost always incorrect and it is always inefficient.
Indexing by byte is rarely useful, but it does have some usefulness since it can be used correctly and efficiently since you can easily find the next or previous character by searching a maximum of four bytes for the a byte that has a MSB of 0. If you want to do something like get a &str that would fit in a n-byte buffer, then byte indices will let you do that efficiently and correctly.