If you can't see what is the purpose of the stakeholders then you're not a project manager.
Stakeholders need to be involved because they are affected by the outcome of the project. Stakeholders want your project to succeed, and they want to know the status of your project (not at a microscopical level though), so that they feel assured that your project is on the right track.
How are stakeholders affected?
Let's say, for example, that you work in a Telecom company. Your project will enhance the IP Telephony service that you already have, making calls easier, which means that your customers will increase, and so will, of course, your company's revenues. Stakeholders will definitely want the company to become more profitable, don't they?
It is not the responsibility of the project sponsor to manage the stakeholders, his responsibility is to look after your needs and help you (and, in one way, manage you). You are the person responsible to report to the stakeholders, otherwise, the communication process will be like a "broken telephone": You tell the project sponsor one thing, and he tells the stakeholders what he understood from you, which might be different from what you really told him.
Better report to the stakeholders yourself, this way, if they have any question, they can ask you directly (imagine them asking the sponsor who has to get back to you to get the answer). This will make communication easier. Additionally, reporting to the stakeholders exposes you to the company's elite, who will help you, and from time to time, offer you opportunities.
I think the funnest part of the project management's work is to manage his stakeholders.
One last thing, the client is a stakeholder, you want the project sponsor to manage the client himself?