While it's true that a lot of DP's served under other Cameramen, they were almost certainly working as AC's or Operators, rather than observing as some sort of Apprentice.
Assisting is a really good way to learn your craft. If you're lucky, you will get to work with a wide range of talented DP's, and you will learn a lot. You just have to remember that you are there to do a job as well as learn.
That path doesn't suit everyone. A good friend of mine who shoots commercials, was never an assistant, and quite freely admits he would have been terrible at it.
Stuart