I'm not sure if it applies to Mellotrons but it has been my extensive experience that multitracks are noticeably less noisy after a thorough, properly executed demag. I'm sure there is plenty of back and forth as to the science of whether it is required, but to my ear, there is an audible difference, especially at 15ips.
I can offer no scientific explanation other than, to my ear, and my tech's, it just sounds better.
After reading the post, my tech replied to my query with the following:
"What can I say... Tape machines sound better when de-maged
this guy should have tried using a soundsawesometer when conducting his research."