I use em', they're great on close range maps or for helping get a volk squad outa green cover while a riflemen engages it from afar(I send them around the flanks, not towards the volks for those wondering). Primary reason I love having one of them is ofc demos. Planting early demos 2-4 mins into the game can really tip it in your favor if you place it well. I agree they're harder to keep alive late game, and are really only good at defending your back cutoffs or repairing/planting demos during that period of the game. Overall they are a good investment imo.
Pretty much what VonIvans said. I love using them on Urban maps like Semoisky and La Gleize. I use Demos nearly every time I gathered 90 munitions --> easy squadwipes if the enemy garrisons buildings. They proved to be very useful in the WFA opening cup.