There is a purpose to copying good cartoon drawings: it's not just to be able to make a good copy of something that's already been done.
It's to learn the underlying principles and procedures that went into the good drawings you copy.
These are good copies, because they use the same procedure that the original artist did.
If you truly understand the underlying tools of good cartoon drawing, then you should be able to make your own poses using characters you have learned to copy well.
John studied and drew the comics above (and more of my exercise suggestions) and then tried his hand at creating his own poses of the characters:
These tell me that he didn't merely copy by eye. He used his head to figure out how the drawings were achieved. He learned the principles and then applied them to his own creativity.
This I think is missing in cartoon schools, especially when it comes to life drawing classes. The schools encourage you to do certain exercises - like life drawing, but don't encourage you to learn anything from them that you can apply to your own drawings.Some people are good at copying things that are in front of them, but are lost at making original drawings look good.
John is doing the exact right thing. After you learn something through study and copying, then APPLY what you learned to your own poses. It helps to use characters that you have already learned to draw from the copies, not to design your own - because your own designs may have built in flaws.