An unpwered hob relys on the PC power supply, which with some devices -a scanner say can be considerable and affect the computers performace. It does not matter too much for card readers or mice etc.,
More over even here if devices not being used are left plugged in they will all be havine there 'pull'
and can actually stop the msachine or at least not read.
The rule here is if you are not using it pull it out and if you can power it separately.