Cable as in the Directv cable or cable as in Cable TV? With satellite, while a multiswitch may lower you reception strength, it will not lower the quality. A picture with a signal strength of 50 will look exactly like a picture with a signal strength of 100. It's either on or off.
Now with cable TV, if it's still analog cable, then yes, it probably will. My suggestion would be to either bypass the multiswitch or goto Radio Shack and get a powered signal enhancer.