I do occasionally use them on the couch or in bed, but for anything productive... I don't really ever want to work with just a laptop screen (13/15/17 inch - who cares, the form factor sucks for this).
So in that case, the 13 inch machines are great because they're light and portable. Much prefer the 13 inch XPS to the 15 inch XPS. It fits in many more bags and the only real component downgrade is the GPU, which I don't really care about in a work machine anyways.
I also find - surprisingly, they tend to get better battery life, simply because they aren't powering as much screen real-estate (and are doing it without a discrete GPU).
So yes - the 15 inch really is a lot harder to carry (I own both - older gen XPS 15, new gen XPS 13). It weighs literally twice as much and requires a much larger bag.
For the other 20% time when I'm on the go, I want the smallest , sturdiest laptop I can have, with the best battery life . I've got a Dell latitude. Although Linux is just BAAAAAAD wet battery life. I can't stand windows OS though.