While I once used shelters 90% of the time in the 70's and early 80's I no longer use them. All the reasons others have listed. But the question was why would someone use them. The two reasons are to avoid carrying a tent and to avoid tenting in the rain. I don't mind carrying a tent and don't even mind tenting in the rain. I do hate packing up a wet tent and putting up a wet tent in the rain. Shelters are great for breaks in bad weather and good for emergencies.