The Pres. of my homebrew club, who is more familar with the SoCal beer scene than I am, has talked about SDSA's as "1.040 and 50 IBU's," with the implication that they are "Sessionable IPA's" and that the malt backbone has to be relatively big to stand up to the hops. It seems, from my reading about Levitation, that coloring it up to copper/red with dark malts is done (not sure why)... so I figure the style is something in between a bitter, an American Amber Ale, and an IPA.
I'm not really sure how prevalent this style is in SD, but I get the sense that many or most of the brewpubs down there have one on tap most of the time. I mean, it makes sense, though. They can presumably be turned around really quickly, and they use less malt, so they are probably cheaper to produce than a lot of styles and they provide a smallish brewery with plenty of healthy yeast.
From my perspective, and in making the decision to brew one of these - it came out of a fall and winter spent brewing too many bitters and ESB's. I mean, I like bitters, but I like IPA's, too. And I loved a pale 1.044 bitter I made last summer that was bittered to about 38 IBU's, fairly aggressively late-hopped with Willamette and Nugget and dry hopped with Willamette and Cascade... Back them, the thought occurred to me that I could always just add some domestic hops at 5 min, 0 min and dry to get my bitters where I want them (style guidelines be damned!). But upon reading and hearing about SDSA's, I figured I would give it a go. I don't know if I am sold on a beer that small and that bitter, but I have heard good things about them, and it is always fun to brew something and then get to drink it so shortly after it is made.