The sequence will end someday. Somewhere in the year 5000, and the same goes for your MD5, it too can hold X values and "that's it".
The main reason for not using your own primary key values is that you'd have to make sure that the value you generate is indeed unique, that means
either selecting to see if you can find a previous version of that value, or forcing a unique contraint and checking for errors on insert.
Auto_increment does these things for you, it will never ever be a duplicate.
