Good reason? Let's say my primary alias is Gmail, which I want to delete and only use Outlook. Fine, hard reset and... all my saved games are deleted, cause Xbox LIVE games on WP8 don't have cloud saves. Great option.
It works exactly as designed. Sure it could use improvement - the aforementioned game data saves, but they're not going to change the need for a hard reset to change primary accounts. Nor should they.
Why? I'm not forced to format my PC after changing primary alias of MS account, so why should I have to reset my phone? Without full cloud backup it's a problem.