>>If the index is PK index you need to drop constraint not index.
>
>Until now I haven't used PK except when I have an autoincrement field. What is the advantage of a PK other than assurance of uniqueness? Are there performance gains?

If you want to create proper normalized design for your database, then you need to create primary key in every table.

Please take a moment to read this blog post
http://blogs.lessthandot.com/index.php/DataMgmt/DBProgramming/best-practice-every-table-should-have-a