If there's no bugs, creapy-crawlies, plenty of trees, and you have plenty of time on your side, a tarp is adequate. However, if you're camping where there's problems with mosquitos, black flies, midges, scorpions, rattlesnakes, etc., then I'd prefer to get a screen between me and them. That said, I use both. A good silicone impregnated nylon 8x10' tarp weighs in at about 10 ounces and makes a great place to get out of the rain where you can cook dinner without making your tent smell like dinner (something to avoid in bear country). It gives you a back up shelter if your tent is damaged or you discover it's not as waterproof as you thought it was. They don't come cheap ($50-75 for a 7x9-8x10'), but because they're smaller, lighter, and cheaper than a tent, I keep mine in my car kit (BOB/GHB) along with a one-person mosquito net.
Full disclosure: Sometimes I want to pack fast and light and carry just a tarp and some 550 cord for my shelter, but I avoid anywhere near water when I do so I don't get eaten alive. It's a great backup shelter when you're planning on sleeping out under the stars in the desert, just in case that chance of an afternoon thunderstorm turns into a reality.