I myself chose to opt out and not sign the agreement with the consulting firm because:
1. they changed their tune from 25% to 100% travel (I actually like my wife so that was not acceptable to me)
2. after 6 interviews and me telling them repeatedly what my base salary requirement was they undercut my base by 25% with their offer
3. their non compete was ridiculous

So instead I took a new contract with a firm overseas for a few months as an independent (it just happened to land in my lap at the right time) and never looked back. The thing is, with the financial down side, the stress it would have put on my family and the noncompete risk, it would have been a huge mistake.

But this whole discussion begs the question... if consulting firms are so stupid and evil with their high overhead, bad consultants and satanic sales forces... why do they exist at all? Are there just a lot of firms out there dumb enough to hire them?