Trim (as in DIR horizontal trim) is so much BS anyway. It's rationalized as making for better deco, a concept that there is no data to support. Submersibles dive in DIR-like trim and go up and down due to the mechanics of their ballast tanks and clump weights, organisms in the water point their head in their direction of travel and then go, adjusting their buoyancy to hold station not to arrive on station.
I don't judge a diver based on his or her ability remain flat, that's just an artifact of moving weights around on the body, hell ... I can set up just about any diver so that they're not only flat, but so that they can't be anything else.
I judge a diver on his or her ability to take and motionlessly hold just about any attitude in the water, head down, right side down, 45 to the vertical, etc., even plain old DIR-flat, but only as part of their box of tricks.
I never put much stock in the flat trim==good deco honestly, even though I have heard it bandied around.
I think we have to remember that most reports we get here, and most people (sort of by definition) are likely from DIR-F classes where you have to start somewhere and for open water, flat trim is a pretty good place to start in general, and so this I think is why you see a lot of DIR-F divers focused on it.
Definitely it is better to be able to hold any position in the water.
I think that one thing I have really benefited from is having good mentors, because if you do just take GUE training in isolation, you get a narrow view of certain things (somewhat out of time-based necessity).
Also, with all the bashing backwards and forwards, Lynne is probably somewhat justifiably concerned that she will catch some stick for being "out of trim", and the classes she has been in probably have harped on about trim especially as trim (and by extension) positioning in the water is often harder to "get" in doubles than singles.
Heck, my trim in doubles is far from great, and it's been harped on about in Tech1, Cave1 and NAUI trimix1. I just seem to have a head-heavy "body type" and being short, not much "lever" with my legs so it's harder to maintain a position. More diving seems to be helping but it's an uphill struggle.
IMO there is nothing wrong with being head up/down or whatever is actually needed for the environment, but for general swimming along a flat bottom in cave, OW or wreck, flat trim definitely seems more streamlined and give you a good non-silting position.
I have exited Mayan Blue in MX, and it was almost a perfect PADI-style vertical ascent as that is the only way I saw it as being possible to make the entrance (and I still touched stuff).
Maybe a Cave III instructor could do it horizontal and I would love to see that