Yes to use HappyPool you need to have a worker active to setup the mining software. I haven't tried to add additional workers because it didn't make sense to me to have multiple copies of mining software running on one PC it would bog it down.
When would you use multiple worker accounts....Perhaps if you had a lab of PC's or some older computers around your place. They could be used to mine all at the same time you could setup different worker accounts for each computer, configure them properly, and have them all contribute to the same HappyPool master account login. Then multiple pc's could mine coins all at the same time. I haven't tried that yet but think it should work because each computer doesn't need to know about other ones. It isn't in the setup run file configurations, so they are all independent. Computer A doesn't depend on or need to know if Computer B is mining for the account in other words.
It would be similar to say you had an oil field and each computer would be one of those well pumps that send it to the container.