For a concrete example just look at Robin's recent Malthusian posts, which I (simple-model-skeptic) find utterly ridiculous due to their reliance on model with known false assumptions, conflicts with a lot of empirical data, and great uncertainty about the future, and Robin (simple-model-lover) basically says because we don't have any better model that's equally simply, this one must be true, and can be extrapolated as much as we feel like.

I tend to agree. I've got the distinct impression that the dubious assumptions you've mentioned are motivated by orthodoxy more than accuracy.

I find it useful (or at least interesting) to explore what things may be like if we follow certain assumptions to their conclusion. Robin's 'burning the cosmic commons' analysis is an example I gleaned insight from. However when I am using such models to make actual predictions about the future I take far more care in my assumption selection.