Broken tachometer? so what
 

Hired a twin the operator of which informed me the tachometer is unserviceable, however he insisted that was ok and to depart anyway. As per the POH the tachometer IS required.

By the way the operator is a senior flight instructor.

Has anyone had similar experiences? Would anyone depart with a broken tachometer?

Unfortunately, more common than it should be in our world of aviation. At least you were renting. Plenty of guys are told "you fly it or you're fired!"

In the US an aircraft is not considered airworthy if something is broken unless it is an aircraft with an FAA approved MEL (Minimum Equipment List). In that case the MEL spells out what is actually needed for flight and any restrictions. For example you might be able to fly with prop heat inop if you are not flying into predicted icing conditions.

Not sure where your issue took place and I'm not familiar with rules outside the US. Plenty of FBO's here routinely rent aircraft that have broken equipment. If something occurs the pilot will be violated by the FAA for departing with inoperative equipment. So unless it's something trivial I'd use caution.

Most (pretty much all?) rental aircraft will not have an MEL. Although I have no personal experience getting an MEL for an aircraft, I understand it's a rather involved process (involves FSDO, etc.) and must be done specific to the aircraft registration (e.g. a flight school can't just get one MEL for all of their 2003 C172 SPs, each one has to have its own specific MEL).

For most of the Part 91 rental type stuff, 91.213 applies. Basically you have to have the equipment required for VFR Day and or Night/IMC ops. Plus all of the equipment listed as required on the aircrafts equipment list (usually stuff like stall horns, etc.).

The inoperative equipment also has to be deactivated and placarded as "inoperative".

So "TOMATO FLAMES" + Req'd equipment on the master list (for day VFR anyway)!