Hi,
pmcke, with all respect but I have to disagree strongly.
I'd like to suggest that if you have been wet under a tarp, you either need to learn more about using it, or it was simply too small for however many people were underneath. A tarp that is properly pitched according to the conditions and is of sufficient size WILL keep you dry in about any weather.
Ironically, the number one out of several reasons why we switched to tarping a few years ago - and never once looked back - is because it keeps us magnitudes drier than any tent can.
Why? Because due to the lack of condensation problems - which ANY tent has, as even if you don't see the drops running down the inside, your clothes still get damper over night - your clothes and sleeping bag are exactly as dry and therefore warm in the morning as they are when you go to bed in the evening. Additionally, it allows us a luxurious amount of covered area, which means that we can keep wet rain gear, pack covers etc. separate from the things we want to keep dry easier and more reliably than we could in the enclosed space in a tent, again leading to us being drier. A side effect of preventing water from "spreading" throughout your gear by contact in the pack and elsewhere is that you also carry less unwanted water around with you. On top of that, if you don't accumulate moisture in your sleeping bag night after night, you get away with a lighter sleeping bag as well, because you don't have to choose it so it will still be warm enough after x nights worth of accumulated moisture.
A tent is a few degrees warmer when you crawl inside in the evening, yes. After two or three hours, they break even because the dampness inside the tent WILL reduce the performance of your clothing and sleeping bag over the course of the night, and by the time morning comes, the tent is clearly colder.
You don't need a bivy bag at all if you use your tarp correctly. In fact, it is counterproductive, since it prevents the moisture from getting out of your sleeping bag.
Where there are loads of insects, i.e. potentially everywhere in New Zealand, it is advisable to hang a simple mosquito net - fully enclosed, with a floor, else they crawl in - underneath the tarp, which we do. Works like a charm and weighs next to nothing. We made ours pretty large so we can use the whole space that the tarp provides in the 90% or so of all nights where there is no dramatic storm, and in the remaining 10%, we pitch lower and / or move the net tent away from the centre of the tarp a bit, depending on wind direction. We use a simple draw cord closure as a door, works brilliantly, no zipper noise, can't fail, and you keep most mozzies out as you only make the "door" as wide as you are when you go in or out.
We use tarps in spring, summer and autumn and even on the easier winter trips. Yes we usually camp below the bush line, but we would do that with a tent, too, it's just warmer there and more pleasant to be in most cases. During the day, we go as high as we want to, and if we would get caught by a sprained ankle or whatever, we would not have a problem to spend a night or two up there as well. We sometimes camp up high just for the views if we feel like it as well.
Tarping is a bit different from tenting. You learn after some time which places are better suited and where to find them, how to best pitch the tarp for different conditions and sites, and some other things. But that's no different from tenting - you learned a handful of tricks and skills when you started tenting as well, and you will be way more proficient in tenting now than you were when you started.
We have used tarps in truly terrifying weather - the type of thunderstorm where you don't know if you'll drown, a tree will come flying to kill you, the avalanches may get you even though you chose your site carefully and well out of the way, or if it will just be plain ordinary lightning strike, and sleep is out of the question - and the tarp always worked well and we were dry. In fact, quite often the ground underneath the tarp is visibly drier in the morning than around us, because it dries out over night.
Unless you are talking about full-on winter trips or actual mountaineering, maybe give the tarp another chance... after having bought so many tents, you can hardly justify not buying at least one to give it a shot on a few overnighters, hehe! (Zero criticism implied, by the way.)
Cheers,
Matt