Dnath: If you use a single-coil cartridge in a stereo system, chances are good that you will end up with hum. If you have a dedicated mono system - only one channel from head to tail - a single coil cartridge will work fine.
The key issue for mono playback is not the number of signal coils - but their angular orientation, which determines whether they are sensitive to vertical groove modulations or not. A coil design that does not pick up any vertical modulation in the first place gives better sound than picking up the vertical modulation, then attempting to cancel it out later.
IME, what works best in a normal stereo system, is a cartridge with two mono coils. This eliminates any sensitivity to vertical groove modulations, yet avoids the hum issues that a single-coil cartridge may be prone to.
hth, jonathan carr