Exactly what you said. For example, i once had (am talking about 9.04 or 9.10 here) a problem where the root partition got all weird on me. Since i had the boot partition in the same place as everything else i could not boot and try to fix anything. After that i learned that i should have the boot part separate from the rest since in some extreme cases if something goes bad, it will go bad only the specific partition. I also recommend to have not only the partition of boot separate from others but the home also separate so in the event of updating or reinstalling you do not need to do a backup for all your things.
It also helps to have the boot part in another partition since if it happens to malfunction you know you have everything in another partition safe from harm.