Is polygamy fundamentally immoral/wrong/harmful?
On the face of it, no. It can lead to issues totally unique to itself, but not entirely different from those in monogamous relationships.
Apart from religious rules/laws, is there any moral reason why polygamy should be illegal.
Between consenting adults in a free society, no.
Is polygamy legal or illegal within your religion?
Formerly, it was legal and not abnormal. As of late, it faces a
de facto ban throughout Messianic and traditional Judaism, though it remains to a limited extent in the Islamic world.
Do you agree or disagree with your religion on this issue?
I agree to an extent - it can be an issue, especially if it leads to strife with other faiths (as fCoJCoLDS) As I do not wish to cause strife within my own community, I would not move to change it without a significant change in status quo.
Does polygamy have any benefits?
I suppose it can, as much as it can have negatives. I do not know what these are, as I do not know any polygamous people personally, but I suppose, theoretically, it could provide for a more stable home situation (for example, in a marriage of three people, two could work and the third be stay-at-home, or what have you). But I would not want to draw conclusions without direct experience.