Turnbull and Woltz both make some really good points for both sides of this discussion.
What bothers me is that Michael didn't even bother trying to talk to Fredo even before the senate hearings began. Surely, as pointed out above, Fredo had been back to Tahoe for some time. Yet Michael did not bother to try and get any information out of Fredo, at least not until his back was against the wall with the sensate hearings.
If I found out that my brother was the one who betrayed me to my enemies, I would at least want to know what went down, who approached who, and what kind of offer was made. And I definitley would have tried to find out from him who opened the drapes!