No; you do NOT want to use the "Jack" mode for anything other than jacking and transporting. To get over ramps, steep driveways, etc., just set the active air suspension to Very High. You still want the car to be able to dynamically level itself, though, so don't use Jack.
Quoting for context, not to disagree.
The language is repeated that 4.5mph will cancel "JACK" mode, further reinforcing that it's not intended to driven in that mode.
I didn't catch any language in the ms_owners_guide.pdf relating to what speed deactivates "VERY HIGH".
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I still don't understand why lowering the car helps with loading/unloading passengers.
I think of it in the following light.
If you have a roadster, you might want to raise the vehicle to load/unload passengers.
If you have a pickup truck, you might want to lower the vehicle to load/unload passengers.
At highway speeds, the Model S is lowered. At low speeds, the Model S is raised to improve clearing on poorly maintained and/or uneven roads.
If you "paused time" on the freeway to load passengers, it would be more like a roadster -- raising desired.
Under normal conditions, you'll pick up passengers after having rolled to a stop from low speeds, it's more like a pickup truck -- lowering desired.