I see your point, that perhaps novels have some variations to short stories. This is true. The reason I would say short stories help hone your writing skills more than novels is you can step away and analyse yourself without the pressure to continue writing the rest. Also, short stories and flash encourage - and teach - concision. The more you write, the harder it may be to be critical with every sentence you write (especially at first). Short stories also allow you to experiment without a huge amount of time being put into the piece. To each his own, I suppose.
Short stories are a nice experimental ground for me. I have a story in my head that I think if written well could be entertaining. I have the characters but I'm new to writing and don't want to waste those ideas initially. I have written two short stories that I experimented a lot in. Next thing I want to practice is dialogue. After I have written a couple of short stories I will try to write the novel.
