THXs 2.4 is based on screen height and good 1080 material to give good immersion and image quality - it's probably the crossover point where they feel sitting closer may start to reveal image artifacts, but it also depends on a few things like if you prefer the immersion over the occasional instance where something in the image may be a little more noticeable than if you had sat further back. People like Mark prefer closer than the 2.4 x SH suggestion and are more than happy there.
Image quality and immersion are both important and there will always be a point where the image may appear not as good when compared to sitting a little further back. That's when it becomes a more personal preference and why these things are just recommendations, but they're recommendations based on testing things. Back in the 50s, Fox suggested that 3 x SH was the optimal point before film grain, scratches and projector mechanics would start to become more noticeable if you sat closer, but since then film grain has improved, and Digital has removed many of the issues film and projectors presented with respect to image quality. Now we have pixels and compression artefacts.
If you zoom for scope, then the screen height effectively changes, and so might the crossover point where the seating distance is now effectively closer than it was before - zooming is like moving your seating 33% closer. But again, it's down to what you can see and what you prefer. There are plenty of people that zoom for scope and don't have any problems with it, so it's something you can experiment with to find your own personal preference. I like sitting closer but a brighter image tends to highlight image artifacts more than a dimmer one, so I set up my image to be similar to the brightness you get in a commercial theatre. For me it looks more cinematic and you have less chance of seeing any issues, even with poorer transfers.
I think like many things, using the guidelines is a good way to get into the ballpark, but testing for personal preference is the best way to get what really floats your boat