I have been using a MLD Tarp & Bivy combo for years, but I am considering getting a MLD Duomid or some type of MLD Mid for situations of prolonged bad weather/rain.
One of the things I found using the tarp and bivy in the rain is that I simply want to sit up and have room to move around.
On a trip on Mt. Whitney, I used a MLD Bivy and Tarp and my friend, Cameron, used a DuoMid.....13 hours of rain with winds that shredded other shelters, we both did fine.
I was amazed at how well my MLD silnylon poncho tarp...have total faith in Ron's workmanship after that long night of howling winds.
Anyway, I was really jealous of Cameron's ability to sit up and stretch out in his Duomid, which I was left with tossing and turn under my tarp.
There was only so much laying on my side, back, front that I could take...being able to sit up and move around would have been a nice relief in a Mid.
Just something else to consider between the two types of shelters.