The plus to this setup is, obviously, the weight. As you note, you only need pack 24.5 ounces to sleep two. However, the downside is that you're vulnerable to bugs, wind, and cold temperatures. But if you can work around those issues, and I can think of plenty of situations where you could, this certainly looks appealing! I'd be interested in hearing from people who have tried one of the Shires Tarptents.
Lots of makers are hopping on the ultralight bandwagon, so you have plenty options. I have a mild preference for "complete" tents, because with the super-ultralight tents you usually end up adding a pound or two in extras (i.e., ground cloth and so on) for anything less than ideal weather. Three leaders in this category: REI's Coupe, weighing in at three pounds, 13 ounces and costing $169 for a true two-person, three-season tent (www.rei.com); Marmot's AT ($179; www.marmot.com), a single-wall tent that sleeps two and weighs three pounds, eight ounces (I've used it, and it's great); and Mountain Hardwear's new Waypoint 2 ($250; www.mountainhardwear.com), which sleeps two and weighs just over three pounds.