If you want just one tarp, and don't mind the itty bitty extra bit of weight, a SWT sized tarp could be just the ticket for all year use. The ends can be pulled back out of the way for warmer and nicer weather (just use a bit of bungie or cord and tie each side's end tie-outs together on the inside of the tarp (hope that makes sense). This way it only needs 4 stakes to deploy, instead of 8. The tarp resembles a hex tarp done up this way.
It obviously won't cut any weight, but with a tarp this size you've got a full coverage tarp ready to deploy in the event of some nasty weather. It's nice having something large enough to do your camp chores and congregate under when it's raining cats and dogs.
I would obviously encourage you to check out Backwoods Daydreamer Gear tarps, but the SWT is advertised at 16oz. That's incredible for a tarp this size. I don't know how Ed does it. My Four Season, which is fairly similar in size weighs 19-20oz (stuff sack included) - his sil must be lighter, because there's no way I've got 3+ ounces of hardware over the SWT, even with D rings and longer webbing loops.
Personally - I like a bigger tarp even in the summer. I like being dry and am no fan of being cooped up under a tiny tarp in a good rainstorm.