This has been conventional wisdom for near 30 years. The cross section of a tube has much more torsional resistance than a solid beam (or rod for that matter). The notion is that since there's a single rear pivot point with either hairpin or split bone it requires more flex in the axle to compensate for the limitations on twist of the locating members. Since 4 bars have matching spaced pivot points front and rear, the supposition is that there is less twist at the batwing attachment.

There was an article in I believe Street Rodder 5 or so years ago that attempted to debunk this "theory". That being said, most folks don't want to temp the fates so follow the "conventional wisdom", though some have used a tube axle and either hairpins or split bones and not had a failure. Luck, or proof? Use your own judgement.