MR16 lamps are the most common downlight found in 12volt systems, utilised for their compact form factor and high light output. The traditional halogen light source produced huge amounts of heat, which was offset with improving technology such as infrared coatings or IRC. These heat implications required that those halogen downlights systems in insulated homes needed heat cans to prevent fire.
Nowadays, LED technology has been shrunk down to provide retrofit solutions as an alternative to halogen, providing higher efficiency at lower energy consumption and above all, less heat. It isn't without it's flaws however, with its complicated electronics, LED retrofits tend to have compatibility issues with certain transformers and dimmers.
Light guys recommend that you try a few before outfitting your whole house, or better yet, check compatibility charts before purchasing