I agree that retail is good for honing your people skills ... that you learn a lot about people by selling. That can't hurt a bit ... any good life experience is helpful in almost all careers. But it doesn't qualify you for jobs in other areas of fashion.
I still maintain that you need a lot of other things going for you and the most important of all is contacts ... which means you should work where the people, who already do what you eventually want to do, work ... at the magazine, at the fashion house, at the P.R. firm, the ad agency, not at the retail store.
Marketing and advertisng don't usually have you in contact with the customer anyway ... you are dealing with concepts, research, budgets, media, copywriters, art directors and everything else behind the scenes to produce the right product and then to publicize it. You create the demand for the product and motivate customers to visit the stores.
Then, the salesperson must sell it to whoever walks in and they can only sell what they have been given to sell ... the store inventory.
Selling is all about qualifying (asking questions to find out who is a serious buyer and what the budget is), your presentation of your product (promoting benefits not features) and knowing how and when to close the sale (when the buyer gives you buying signals), and knowing how to kick it up and cross sell other products on top of what the customer just bought. As a successful saleperson, you can take those skills into any other industry and you can make a lot of money in the right job .... but you will still be a saleperson.
