Yes there is a diminishing return. But still a return.
As Bert mentioned dithering helps a lot. You also have to consider your site light conditions and your camera. Some camera have a big read noise some don't so the more subs you take the more read noise you'll add. From a light polluted site you won't be able to expose for very long before your data is swamped into the background sky glow and noise. So it's ok to shoot a lot of shorter subs even with read noise as it is insignificant compare to background noise.
At a dark site though you can push exposures to 15-20min and not suffer from sky glow. So then because you have no background noise, the read noise of your camera will start peeking through. So the longer the subs and the least of them the better.
There are also some critical steps prior to do data rejection when stacking. Calibrating (dark/flats/bias) your subs is important but most of all you need to normalise them. Effective data rejection will only work if you do this. Then there are also a few different algorithm used to register your subs that can improve noise rejection while keeping details such as nearest neighbourgh.
Those are just a few pointers. The topic is far too vast to summarise in one post.