Being seated works well enough for cockpit games for immersion - after all, your virtual avatar is seated there. It's even better if your real life control method closely mimics the virtual one. A HOTAS for flying. A wheel for driving. You can even begin to ignore the HMD weight and FoV limits as being a helmet. For everything else though, yeah, being seated firmly grounds you to reality. It's even worse if your viewpoint is supposed to be from a standing position while you're seated, or vice versa. The mind notices the mismatch and goes "something is wrong here". I can actively ignore it, but it's just that. Active suppression.
That said, standing and motion controls aren't a panacea for immersion. At least not in the "damn, I forgot this isn't reality" level of immersion. With the Vive, the vast majority of the time I'm aware that what I'm viewing isn't reality. It isn't a distracting thing, but it sort of sits in the back of the conscious mind. It's like a halfway point where I know it isn't real, but I'll still be surprised when true reality creeps in (such as accidentally knocking something in real life with the controller). I'm not tempted to lean on virtual objects, as I know they aren't real. Still, I've found myself attempting to step over and under things while in Fantastic Contraption, and I'm hesitant to ever intentionally clip through "solid" objects. It's like I know it isn't real at a higher level of thought, but it's real enough that my subconscious reacts to it as if it was.
