[Steve_Baker]

I don't think it follows that polyandry is less advantageous for a woman than polygamy -- more men equals more resources. I think the reason is that most societies are patriarchal rather than matriarchal and this has traditionally led to men being the greater holder of wealth, and the disparity between the rich and poor being what it is, 1/100th of a rich man is worth more than even 100 poor men. That still doesn't mean that a woman wouldn't choose to have 100 men if she cannot have a portion of a rich man, but in a patriarchal society, those women would likely become prostitutes -- the (to men perhaps) more socially acceptable form of polyandry? This might even be preferable to the woman, since I could see multiple men in the same household potentially being much more rife with conflict. I wouldn't be surprised if societies tend to try to suppress polygamy at equal or even greater rates than they do prostition.

Basically I don't think polyandry is rare at all, it's just in the form of uncounted prostitution. I'd lay odds that you'd see more (monogamous) polyandry in societies that are more (very) matriarchal and/or the distribution of wealth is flat, both being rather uncommon.