Having a tent to sit-up in and hunker down if needed is awesome.
My Thor 2p worked great at Olympic national park in Oregon where I did a 9 day in the rain-forest car camping. Had a windy/rainy storm one night and the tent was bomber, no issues, even with small branches falling on the tent.
But my bivy love comes from the weight savings while being well protected during the winter. The light weight tents i've seen don't appear to be able to handle themselves. If I were caught in a snowstorm would these tents be bomb proof enough?
I'm just worried about gusts of wind in the storm knocking over a 3lbs tent. I don't trust a tent that is set-up by trekking poles. That same tent may work for most of my trips, but on mountain tops is where I question them (openly exposed to elements). But I am in the market of a light weight tent, one that I can trust as my all-rounder. Then I can use the bivy in the extreme mountain tops. That is my thinking atm. Also I've never used or owned a tent aside from the Marmot Thor. I've only used a bivy while in service. I wanted a strong tent and the Thor is strong, but to heavy to backpack with.