Today I read something by Dr Ron Shimek (who may be out of vogue) and he goes into it a bit. If you're interested:
<a href="http://www.reefs.org/library/members/r_shimek_042799.html" target="_blank">http://www.reefs.org/library/members/r_shimek_042799.html</a>
He sees them as natural and necessary, but this is a 3-yr-old article.
As usual, NM Reef states his point with eloquence, and makes good sense I think...
AJ