They should be kept.
A) They provide prize cars
B) They contribute towards 100% completion (Gold ones)
C) They are a means of learning to drive and getting used to the new physics engine
D) They provide a sense of achievement, especially for the more difficult ones for gold
Apart from the coffee breaks, they are both a useful learning tool and provide a challenge to reach gold for the more experienced racer. For casual players, it's easy enough to get bronze on every test, and they do help you improve no matter what your skill level.
They provide an element of realism into the game that most other games simply overlook. You need prove you are of a certain skill level to enter certain events in real life.
They do not need re-thinking, they are an integral part of the game. There is no tutorial in GT because of the sandbox nature of the game. This is where you go to learn your skills and try out different types of vehicles.
One thing I will note is, the information could be more helpful. It could teach you to gradually apply throttle and brakes smoothly (Something which I never learnt properly until GT5
) rather than telling you about the slowin/fast-out philosophy on every test.
For me it's not the difficulty, since I've golded most of the licenses in GT4. It's just the sheer amount of tests that becomes tedious after a while IMO.
You don't have to do them all at once
Personally I like to get 2 or 3 of them out of the way before I even buy a car (I'll certainly need to with the new physics engine). I then do the other ones later to reach the later events. Sometimes though, it can just be fun to try out LMP race cars in the S license right at the beginning of the game. Infact, the licenses in GT4 sometimes affected my choices when buying cars