If you want to use a website to represent yourself as a professional writer, I would suggest not using some free host full of easily accessible competitors. It is easy enough to set up your own site for just the cost of a domain name.
I don't think there's really a good reason to have a writing portfolio, unless you're showing technical writing samples. You don't want to put your work out there before it's published, because then publishers will consider it published. One reason I like dA is that you can "store" your work and you can "hide" it from search engines (by making it viewable only to dA members). This way, you can get feedback and critique, and workshop the piece, and when you're ready to submit it you can put it into storage and not worry about publishers seeing it and thinking of it as published. Does that make sense?