I think it's a weight to comfort ratio problem. My first Thermarest was a full length, thick one. It was very comfortable, but it was also heavy. I had other items like that I brought on my first long tour - comfortable but a little heavy. It all added up. My rig weighed a ton and I ended up with a serious spoke-breaking problem the spoiled the second half of the tour.

Before my next tour a couple of years later I replaced some of my equipment - including my tent and my Thermarest - with smaller, lighter stuff. I still had some comfort items, but the whole rig was significantly lighter. I had a much easier time riding up hills, and didn't break a spoke.

The second Thermarest was a 3/4 length model. My torso and hips were supported, but my legs weren't. It didn't bother me that much and I felt the weight savings was definitely worth it.

HOWEVER, before my last tour I bought a Big Agnes Insulated Core air mattress. It's full length, more comfortable than either Thermarest ever was, much smaller when rolled up, and I think it's fairly equal to the smaller Thermarest in terms of weight. I love it! You have to blow it up every night, but it only takes me two minutes, and I don't mind. I also don't know how durable it will be. I'm worried because if it loses air it will be worthless, while the Thermarest at least has a little foam inside to cushion me if it fails. But so far, the Big Agnes has been a wonderful purchase.