When to use WHEN OTHERS?

Actually, the only cases you have to use WHEN OTHERS are the following ones:

You opened some resources (cursor, file...) and have to close them before leaving
You want to log all errors

In both cases, the WHEN OTHERS THEN part MUST be ended with a "RAISE;" statement.

Firstly, you have misconception that RAISE inside exception will hide the error.

Note: Even if you add a "RAISE;" statement in your exception block it is not correct as you will still hide the actual origin of the error. See the following simple case:

SQL> declare a pls_integer := 1; b pls_integer := 0;
2 begin
3 b := a/b;
4 exception when others then raise;
5 end;
6 /
declare a pls_integer := 1; b pls_integer := 0;
*
ERROR at line 1:
ORA-01476: divisor is equal to zero
ORA-06512: at line 4

And regarding your question about "bunch of processes". You tell me, how would you look into the issues which are a part of production batch process, and you cannot stop the entire batch process due to one error as they would be scheduled with complex dependencies. So, all you could do is log those errors and later analyze. And that's the reason you need to use backtrace feature to ease your analysis to debug those errors.

Can you show where Michel ever said that " Raise" hides the error?

Note: Even if you add a "RAISE;" statement in your exception block it is not correct as you will still hide the actual origin of the error. See the following simple case:

SQL> declare a pls_integer := 1; b pls_integer := 0;
2 begin
3 b := a/b;
4 exception when others then raise;
5 end;
6 /
declare a pls_integer := 1; b pls_integer := 0;
*
ERROR at line 1:
ORA-01476: divisor is equal to zero
ORA-06512: at line 4

Does the error really come from line 4? But there is nothing at line 4 that can raise a "divisor is equal to zero" error! Now if you comment the exception block:

SQL> declare a pls_integer := 1; b pls_integer := 0;
2 begin
3 b := a/b;
4 -- exception when others then raise;
5 end;
6 /
declare a pls_integer := 1; b pls_integer := 0;
*
ERROR at line 1:
ORA-01476: divisor is equal to zero
ORA-06512: at line 3

You have then the actual line that raises the error: line 3.