I've known what Arnold did since the early 70s. He went with twice a week per muscle off-season, thrice weekly pre-contest.
What's got that to do with john doe?
Cyclers cycle multiple times a week perform in the morning strength-traing and cycle 2 to 3 hours in the evening. Same for swimmers, soccer-players, rugby-players and even bodybuilders. They train two to three times
a day. These are proffessionals.
Until this moment you have failed to explain the benefits of training a muscle multiple times a week. There are only two that I can come up with,
first, targeting a other type of muscle fiber,
second, force an overtraining to a certain degree.
You're free to finish the list. And you can't claim that it's even better to train that much, there are some benefits, but there's not much to gain if you keep the routine most of the year. Simple routines were you workout the muscle once a week have proven themselves over and over and over. And please don't come up again with pro-bodybuilders that train multiple times a week. If you don't work, eat healthy, sleep plenty and suffer from no stress you can train two or three times a day what brings you to a total of over 18. Focus on a couple of exercises and go home again.
Unfortunately that's not possible for most of us.