My concern with not having the ability to adjust tension on the fly (automatically, in response to changes) would be on film that’s super loose or too tight. With the tension hub, theoretically you would reach a kind of equilibrium if you fed the scanner a reel that was too loose/tight. Also, as mentioned above, you can probably find some tension setting that works well enough across a whole roll, but that will need to be adjusted for different gauges and possibly for different reel sizes. What you don’t want is something that makes too tight or too loose a wind on the takeup side. (this becomes especially important if you ever work with film on cores, not reels or splits - you do not want to be handling a loose reel on a core).
We see this a bit with our ScanStation - sometimes on old home movie reels, the wind is really loose because the end of the reel wasn’t taped properly. On a rewind bench we’ll prep the reel and get the tension to a more sane level, but sometimes it’s too tight or too loose depending on how the friction knobs on the rewinds are set. By the time it goes through the scanner and is rewound on the scanner, the tension is right where you’d want it.
It’s not uncommon for us to get reels that have different tension throughout the reel, and you can see this in how the dancer arms move. Sometimes you have a compilation of reels that have different issues - shrinkage or other degradation on some while others are just fine. Or maybe whoever wound it wasn’t being consistent about speed and some parts are loose or tight.
For reference, the ScanStation uses stepper motors for feed, takeup and capstan drive, and dancer arms on both the feed and takeup sidea. The amount of tension applied to the film is preset at the factory unless you get a special option on the newer models that allows you to adjust it from software, or if you go into the chassis and tweak the tension springs manually.
At minimum, I’d run some tests on reels that have very loose, very tight, and a combination of loose and tight film pack to see how this mechanism works.
On the PTR argument – why not just make the rollers PTRs? That allows you to use the same roller regardless of gauge - all you need to do is make a generic hub out of Delrin (or 3D print them) that fits standard PTRs - if you’re manufacturing a bunch at once these would be really inexpensive - just a few dollars each to make the hubs, probably, in a bulk order. They’re pretty simple. Kinetta uses PTRs for all rollers, as does our 70mm scanner.