Excellent points from Frank and Rustle, but I think the equipment redundancy thing is being oversimplified.
I've never had an alternator failure, but I have had a total electrical failure. A spare alternator doesn't help if the bus breaks.
Similarly, probably the majority of AI failures I've come across are not due to failure of the vacuum pump, but rather the instrument itself.
As for fuel, I'm certain that the increased complexity of the plumbing in twins has been responsible for at least as many accidents as it has saved -- albeit pilot-induced accidents!
I have a share in both a single and a twin, and fly both happily IFR. That said, there are circumstances when I'm more comfortable in one than the other!