Welcome to the party.!!

Not quite sure what your asking here.? You say single switch on X & Y but do you mean one switch that travels with the axis. Or are you asking if it's ok to just have one fixed switch at one end and use softlimits for the other end.?

Z axis often don't have a lower switch because usualy you'll have material on the bed with different thickness's and tools in the spindle with different lengths.
If you crash then more likely the spindle nose will hit material, unless thin material, way before you hit the limit and that's allowing for the tool to be embedded into the material.
Really the only time a lower limit is help full is for when your jogging up/down the spindle with no material on bed or tool in the spindle. All other cases then you'll either crash the tool into material or bed before getting near the limit.