Well if you are using a secure password(something with symbols like ! or #), it messing up the syntax of the phoenix miner when you put those in a terminal, so phoenix won't start. I like the option of having multiple different miners, that way you can keep your deepbit main account secure so no one can change your pay out address. At the same time, maybe you are running multiple computers and you would like the option of have one card or computer pay-per-share mode and the other pay out proportional. I really like the customability of the miner. It also provides another tool to monitor your efficiency.

So those are the reasons I thing it exist. You can run multple computers/GPU's on the same worker account. Extra workers are for more flexibility and security.