I fancy a MM at some point, so have been skillfully avoiding work by browsing gear online.

The ultralight tents that seem to be most popular (Terra Nova Lazer and, at the extreme, something like the Nordisk Lofoten 2(!)) don't really seem that much more substantial than a good quality bivvi bag. Does anyone use bivvi bags and a dry-bag for kit on the overnight camps in place of a 2-man tent, or is there some major disadvantage that I'm not seeing? It seems that the cost to weight ratio is in the bivvi bag's favour. I suppose that even a super small tent gives you space to hang out before bed if the weather is horrific, but if the weather is expected to be decent I'm not sure that matters too much.