Hm, should the guest "need" to see a ride vehicle? There is no need to see any ride vehicle before boarding, if the idea supports the withholding or masking as a story element (which in this case, I think Mansion withholds purposely and masterfully). Rise of the Resistance greatly succeeds in this regard, as does Indiana Jones, Space Mountain and Flight of Passage (and Tower, up to a point).
It generally helps to give guests a preview of what they're getting into. You'd be surprised how often guests don't understand what they're supposed to do with boarding an attraction, and many rides are different, and if they've never seen a vehicle like that, you can't blame them for being confused.
Continuously loading attractions also tend to cause more issues than rides that stop. Most people in their average lives don't, say, get into cars that are already moving.
The fact that they felt the need to redesign the Mansion load areas in Florida and Tokyo so that guests have more time to see what they're getting into implies that it was a bit of a problem at Disneyland or something they wanted to reconsider.
As for your examples, the only attraction where that's really true is Flight of Passage, and even then you have a safety video that roughly clues guests into what the vehicle is going to be like and how they need to strap in. Tower of Terror, you're told it's an elevator and it's indeed an elevator, and the restraint is a seat belt, which everyone who has ridden in a car ever knows how to use. Indy, you're shown a vehicle and how to fasten your belt in the safety video, then you see a preview of the vehicle and boarding platform before you go up, down, and board yourself. Space Mountain, you circle the station so that you have plenty of time to see what the vehicle is and how fast you need to move. Those design features are very much intentional to minimize confusion and backups during boarding.
My general life experience has been that you cannot assume that people walk into a given situation knowing how to do anything, and anything you can do to try to mitigate that situation-particularly if you can do it in a way that helps efficiency and is less likely to embarrass a guest-is a win win.