That's a valid question. As the vaporized rock cools it needs to be directed to the annular walls of the borehole being drilled. If it flutters chaotically up t

That's a valid question. As the vaporized rock cools it needs to be directed to the annular walls of the borehole being drilled. If it flutters chaotically up the along the drillstring and sticks itself to the drillstring then you are effectively blocking the borehole as you drill. Even if you rotate the drillstring the cumulative effect is that the drillstring becomes a long length of sandpaper or a vertical grinder, grinding the fused rock from the annulus above where you are currently drilling.