@LarrytheStanimal, I can empathise with your approach and think you have some sound basis for it.
I spent all last winter through my TR plans spinning a high cadence (often 95-100rpm+) in a high gear in ERG mode and got a great FTP growth but at the expense of hill-climbing prowess. I’d ended up with a fantastic cardio vascular system but not the best development of leg strength for pushing up steep gradients. My target events are TTs, many of which are quite hilly.
This winter I’ve actively addressed this issue by
(a) reducing my gearing (to make it feel more like every workout is uphill, which is the effect of the reduced wheel speed and resulting reduced “helping hand” of flywheel inertia) and
(b) reducing my cadence. Although not to the lower levels you are aiming for, it must be said (more like from high 90s to 85-95 in my case)
I think you are okay to pursue an element of lower-cadence work for your MTB goals, but I honestly think that the level of muscular fatigue that you will get when you get to over-unders like Palisade will be too hard to take and you’ll crash and burn and wonder what’s wrong. It’s all I can do to survive these workouts at around 90rpm, and that’s when I’m trying to keep my cadence as low as I can!
If I were you I’d target lower cadence work, definitely, but do it strategically, maybe confining it to the sweet spot type intensities, or maybe just in selected blocks (or part blocks) of the threshold workouts (in place of aero drills for example).
If you don’t do some of the threshold/VO2/over-under type workouts at the intended cadences (usually advised at 85-95 level or even higher for the very high wattage stuff), I believe you’ll be missing out on some valuable cardio vascular developments, not to mention that they will just be impossible at very low cadence.
That’s my two penneth (approximately two cents if you’re from over the pond!), I hope it helps a bit and I wish you all the best with your training!