Maybe you’re right, but I don’t know if I agree about the bad impression: If I drop a tab directly onto another tab and it stacks them together, I’d think I’d discovered a cool feature, not a problem. How often do you want to place windows directly on top of each other in almost the same position, and in the case where you do, wouldn’t having them stacked be useful?
In the end it’s down to what the devs consider the best default behaviour, and given that the aim is to have defaults that everyone likes over configurability, they might not want to have an option to change the behaviour. But anyway it’s food for thought…