I believe it all comes down to this:
If the LDS crowd admits that Joseph Smith's sexual encounters with his 27 (some sources say it was as high as 60) "wives" was sinful, tragic , and immoral behavior - then his whole "prophet" halo is stripped away and they have to ask themselves some very hard questions.
Interesting point. Yes, I can see why that would lead so many of them to defend it even though they do not sanction it today. That had not occured to me.