No matter where it goes,it will be trying to enter a pipe already full.
..if anything logic would say it will create interference.Aside from that
the exhaust can't get out any easier or quicker?
It's NOT already full. It does not create interference.
Exhaust isn't a steady stream of gasses out of each cylinder. They're pulses. Only one cylinder at a time is huffing into those pipes. The gasses head for the exit - path of least resistance. A column of gasses has weight/mass, which gives it momentum. That means it takes work to slow it down. That's why when an exhaust valve closes, the column of air moving behind it toward the tail pipe is still 'sucking' on that valve - until the next one opens.
Having an H or X does two things - it allows the exhaust column on one side to 'suck' on the other. It also splits the escaping gasses into two sides, effectively cutting exhaust resistance in half. That's why the exhaust tone changes - it's effectively behaving as if the pipe diameters have been increased substantially. There's really no reason not to run one of some sort unless one simply cannot stand the sound of a cross-over style system or spend so much time at rpms where they're not adding much (sprint cars, for example) that the expense or maintenance headache (matter of access on some cars) isn't worth it.
It's not done by/for magazines or for exhaust manufacturers to sell more pipe - every major form of road racing uses them. Unless they're drinking the flowmaster koolaid too, huh?