I'm of the opinion that you are required to get an EIN and file a partnership return (form 1065). The partnership does not pay any tax, it is only an information return. Each partner's income or loss is reported to him on schedule K-1, which he uses to report the income/loss on his individual return.
If, instead, each partner simply reports his share directly on his form 1040, the net effect is the same. Many people do it that way because it's easier or they just don't know it should be done differently
