When NOT to be a sole-proprietorship or partnership.
Smallbizsurvival has a good article on the times when you should NOT be a sole proprietor or partnership including:
Ownership – if you want to bring in a partner
Actually, since the LLC has become so popular, there are few if any reasons not to incorporate or form an LLC with the exception of a few states (California) that have a large yearly tax on these entities ($800 minimum). Learn about incorporating or forming an LLC right now.