Pros of sites like Tapas: webcomic cross-pollination with readers and basically free advertising.
Pros of a standalone site: Total control over the design of the pages, what is presented to the reader and how.
With Tapas I can't change the layout, or how it presents pages to the readers, I can't even put in a bonus section with fanarts or character info. I would actually consider Tapas being a secondary platform in relation to my main site, since it is so much more limited in what it allows me to do. I can't even change the background color! Bright white works so well with mostly dark pages!