AquaTec once bubbled...
its all about paying for courses with PADI
why would it be wrong to teach a Drysuit, wreck and Nitrox course all as one combination.
Although I agree that PADI sets up a lot of the things that they do to make more money by teaching more courses, and that their profit-driven philosophies sometimes are at odds with improving diver education, I do think it is a good idea to keep training dives for various specialties separate from each other.
I would not want to have a student working on controlling their buoyancy with a drysuit at the same time as they are doing a wreck training dive. That could be disastrous. If each dive is separate, focusing on developing one set of skills at a time, it is safer as the student is less likely to get overloaded, and can concentrate on the skill they are learning. There are some combinations that might be OK, for example using nitrox while working on other skills underwater, but not at a depth anywhere close to MOD, so that if the student is distracted by working on their other skills, and forgets to watch their depth, there is no chance of them accidentally exceeding the MOD for their nitrox mix. When doing a deep dive (especially for the first time, as in a deep dive for AOW or the deep dive specialty course) it would be unwise to task load the student with other unfamiliar things. I think it might be OK in some circumstances to combine underwater nav with night diving, since visibility is lower in most places at night, but then again, for a student's first night dive they may very likely be apprehensive, and adding other skills to the mix might make the whole thing much more stressful, which in turn might make panic more likely.
If you are doing training in a particular specialty, I think it is almost always best to work on only a few new skills at a time. Add other skills on other dives.