Most sleeving companies do one or the other, and in many cases do both. If you motor is gonna blow, its gonna blow. You speak of a headgasket like it is a fuse in a stereo. It blows to keep damage from going any further. A headgasket's purpose is not to blow when something is wrong and protect the engine, it is to seal the head to the block. Without it, the head and block, which are made of somewhat hard material, would not seal up very well, and have a hard time holding pressure.