because they're completely stark raving nuts, i assume
j/k
no, actually, it removes them from their bound spirits set. so, for example, mr elf shaman the shadowrunner with 8 charisma and limited cash resources who has to worry about getting shot in the face regular will say, "the hell with this nonsense, i'm using my karma to boost myself, i can't afford to keep all 8 bound spirits slots (plus potential ally spirit) full most of the time anyways!" and since he's regularly risking death, being more magically powerful becomes more important than being wealthy.
mr ork corporate magician with a charisma of 2, on the other hand, who sells his services, might run out of bound spirits on a regular basis, and may very well almost never see any combat depending on his specialty. for him, it is worthwhile to turn his karma into cash, because he doesn't have to worry about buying his drain attribute up until it's through the roof, he doesn't have to worry about skills outside his specialty area, and he doesn't get shot at nearly as often most likely. he's basically trading his karma for cash either way... it's just a question of "lump sum now or a little at a time as i improve my skills".
also, there's the fact that they come back after being disrupted... i'm pretty sure that costs you an additional service for a regular bound spirit.