With Skill NO BIVY with Tarp?

Not in my experience. If the weather is good, sure, but if the winds pick up to even a breeze, the warmth is sripped from your bag. This is especially true with a quilt. If the humidity is high or you're dealing with spindrift, the effect is even worse. I find a breathable bivy to be a critical component of my poncho/tarp system.

However, his point about site location is a key one. I tend to be in wet country and at or above treeline. If you choose a protected site, you can avoid much of the concern I just raised. Still, in the northern Cascades, I'll always carry my bivy (unless maybe the weather report is perfect in summer). I've paid in cold nights when I didn't have it from loss of warmth.

Now, when using an enclosed shelter like a Tarptent, Wild Oasis, or a Gossamer Gear Spinshelter, this concern goes away. I'm specifically talking about when using a small ponchotarp.

Doug

Same experience as Doug stated, was trying poncho with various bug screen shelters over the past 3 summers and most always was woken up by a slight breeze or dropping nightime temperatures even while using with a warm hat. Seemed worse when using a quilt and would turn over and have breeze sneak in under the sides.
Breathable bivy with waterproof floor paired with a small tarp/poncho seems to be the best combo so far for the weight.

It depends on the conditions and your bag. If you have an enclosed mummy bag and reasonable good weather and/or high skill with pitching a tarp (closing off the windward side) you can get away without a bivy and adding extra down is a good investment.

With a quilt style bag it is much harder to keep warm without a bivy though in an enclosed tarp it is easier. You get a bit more versatility with a quilt/bivy system and it lets you use a smaller/lighter tarp.

Yeah, I think Brian R used a Feathered Friends mummy w/ Epic shell on his Triple Crown. I can't speak for what he's using now.

I also prefer a thinner less absorbant, more breathable top-bag with a bivy (similar weight) for anything but beautiful weather. But a bivy can be overkill for a lot of the summer too, esp if your bag is truly 3-season rated.