I hate to ask such an ignorant question, but why should a writer care if the agents company is an LLC or under sole proprietorship?
If an agent kicks the bucket and your money is in his/her account, even if it's a business account at the bank, and s/he is a sole proprietor, that money is considered part of her estate. Have fun getting it.
LLC means the business and the agent are two separate entities.
There are other ways to make this happen too, but you want to make sure your money is protected in case of an Act of Dog, Death or other Unfortunate Event.