You have that slightly wrong there, chief. True, skimmers don't block los, but after they crash and burn, they are no longer considered skimmers. They are considered terrain. As terrain, you can still hide behind them. The skimmers don't block los basically just makes it impossible to hide your raiders behind other raiders to protect your wych and hq raiders from enemy fire with your cheaper raider squads. As far as them counting as difficult terrain, that means you can move through them rolling difficult terain tests as normal. Hope that helps.
Your un-hero,
CrimsonShadow