Any "scraping" connection creates some heat at the surface area from the plug and socket contacts.
Additional rising heat from a heater below, will cause the heat to dissipate more slowly from the connection and will add to the deteriation process of the plug and socket and possible premature faillure.
Also the heat on the lead of a plugged in appliance will soften it, add to the resistance of the lead, hence less voltage for the appliance.
There is an other risk of the lead touching the actual heater and early insulation faillure or flashover to the 'hopefully' earthed heater.
