The best answer that I can give you is the same answer that Bob Dole gave when he was asked, "Boxers or Briefs".
Depends.
A watch maker needs very good vision to work on the small parts inside a watch. He might not tolerate any blurring or distortion of his vision. Someone else might not be bothered much by quite a bit of blurred vision.
There are measurements that might give you a measuring standard to go by like 20/40, 20/60 or higher.
Tom, The 20/200 and 20/400 you mentioned are most likely uncorrected visual acuity. You probably are near sighted, (Myopia). and have much better vision with your glasses.
You might consider the point that you cannot pass the vision part of your driving test as a point where you want or need better vision. The real answer is when you are having trouble seeing what you want or what you need to see.
A good physician would consider several facts in determining when to recommend surgery. First they would want to make sure that removing your cataract will most likely result in improving your vision and justifies the small risk of surgery.
They might look at your occupation or hobbies to determine any special needs.
It has been said that everyone that lives long enough will get a cataract. Some develop very slowly and some can be caused quickly by circumstances. A strong electric shock can cause a traumatic cataract. An accident from a high voltage power line or even a bolt of lighting might cause the lens in the eye to become cloudy. That is what a cataract actually is, a clouding or lack of transparency in the natural lens of the eye.
Another thing that can speed up the development of a cataract is certain medications, Steroids for one example.
Steroids have there uses and are very effective, but can sometimes cause that side effect.
The real answer to when it is necessary to remove a cataract "Depends" on what you want and expect as far as the quality of your vision and your needs.