so, lets take two hypothetical tanks (or the same tank during setup and when it's been fully stocked and established for a while).
tank A: just some plants, no livestock. The only source of ammonia is a bit of decaying plant matter.
tank B: plants, and livestock. The decaying plant matter is still a source of ammonia, but in addition, you have ammonia from fish/livestock excretion.
The bacterial population that will establish itself in tank A is nowhere near adequate to handle the filtration needs of tank B, and without adding extra ammonia, it won't grow to the levels needed for B. That's why most people do a fishless cycle, so that they can get the population needed for tank B, without stressing the fish.
I assume what you mean by 'add fish in a sensible way' is along the old manner of fish-in cycling, where you add a fish here, and then a week later add another fish or two, etc. until you reach the desired level. I've done that before, and it worked fine, but there are a lot of benefits to doing a fishless cycle, especially if one is dealing with mail order, or expensive/rare fish.
I'm not doubting your skills or experience, but I think it's too easy for an unexperienced person to read your post, set up their tank with a few plants, wait a couple weeks, and then fully stock it and end up with fish dying due to stress and ammonia.