Cheryl Morgan writes more eloquently than I can about the brief tenure of Jonathan Ross as Loncon 3's Hugo Awards host.
ConJosé had a "Master of Ceremonies" decided and invited (Tad Williams, who did a fine job) while we were bidding. This is not quite the same thing as a selected host for each event of the convention, particularly the Hugo Awards Ceremony; it's just short of being a Guest of Honor (you get a footnote in the Long List of Worldcons for it). Accordingly, the Bay Area in 2002 Worldcon bid committee all had a say in the selection, and it was a multi-valued selection: When we were initially considering candidates, you had four possible choices: Yes, Abstain (which of course was the default if you did nothing), No, and Veto. Any member of the bidding committee could unilaterally veto a candidate about which s/he felt so strongly that if we had selected that person, the bidcom member would have felt obliged to resign rather than appear to be supporting that choice. This is stronger than a No, which said, "I don't care for this candidate, but will accept the will of the committee and work with it if the rest of you really want this person." I devised this system myself, and I thought it led to a slate of candidates that the entire bid committee could support. Those who joined the operating committee after we won the bid naturally knew who the Guests were by then and could not plausibly claim they didn't know with whom they had to work.
Again, I stress that a Master of Ceremonies/Toastmaster selected before the convention as a "headliner" (just after the Guests of Honor) isn't the same thing as an MC selected to host an individual event, and the mechanics are different.
Having had to deal with horrible PR blunders with the 2002 Worldcon makes me sympathetic and less willing to criticize other Worldcons' mistakes. (And Twitter hadn't even been invented yet!) Go read Cheryl. I share her frustration, although I'm not yet so annoyed as to walk away from everything just yet.