When I'm the client, I feel fixed-fee gives the contractor an easy backstop/fallback position - they figure they can half-ass the job and use the additional time to scout for more work.
When I'm the contractor, it's the same, but in reverse - fixed-fee makes clients feel like they can demand the moon beyond spec at no additional cost.
Hourly/daily rates make sure that BOTH parties have something at risk, which tends to be keep everybody in line. The contractor's at risk of the client firing them. The client's money is at risk if they keep demanding endless revisions.
Actually a good lesson for business in general.