I have been unable to find an advantage that vimwiki or vim outliner have over just using vim with markdown in a Dropbox directory. Folding, numbering, etc work really well for me for that. What do you like about it in particular over what I could do with markdown?
There are definite advantages to just doing what you do. I like classic markdown syntax more. Things like #foo instead of = foo =.
But I particularly like some of the ways vimwiki functions.
I like that I can type project_notes and hit <return> and it opens up a new project_notes.wiki, and then I can hit <backspace> to go back to the previous wiki.
There are more normal mode keybindings that you might find yourself using. Sometimes I remove unwanted wikis by <leader>wd, which will delete the wiki you are currently viewing (it's nice in that you can see the content then quickly delete). Renaming, too.
Maybe I can sum it up to just say I prefer the navigation and handling of new/unwanted files with vimwiki?