This is a tricky one. Take all of this with a grain of salt.
The major advantage of using the q tag is that it automatically handles quote nesting
That depends on the browser, and unfortunately not all browsers add the automatic quotes, or they don't add them correctly. The inconsistencies are mostly with older versions of Internet Explorer, and though things have gotten better since their usage has dropped, it's still a consideration for doing the punctuation yourself.
This exposes a shortcoming of your theme, Simpla -- which is that it isn't consistent with how it styles q and blockquote display.
For most, if not all, of our recent themes (say the last 50-60 we've launched since early 2010) we've been using a CSS technique called a "reset" that allows for consistent display of HTML elements across all browsers. Including these two quotation elements.
Minileven uses this technique, also. If you were to use switch your blog over to the latest default WordPress theme, Twenty Eleven, you'd see the same display as Minileven. Where the q element does not have auto-generated quotes around it. Which is intentional, for consistency sake.
The reason I'd argue it's a shortcoming with Simpla is that you cannot rely on generated content (i.e. content added to the HTML screen output by browsers) to be consistent across all platforms and browsers. The reset technique, on the other hand, is much more reliable: you know that all browsers will produce the same output. You know that if you add the quotes in HTML around a element you want see double quotes on the screen.
Most HTML authors would probably argue that they'd rather have control over the output (by adding the quotes into their markup) rather than have double quotes or inconsistent display on their readers' screens.
I won't go into whether or not you're using q incorrectly. That could be argued both ways. :)
For some fun reading, see: