This instal brings up an issue that I often have with lighting installs: how do YOU do it?
I often see a tail of wire just poking through a hole, into the back of the appliance.
While I like the idea of a box in the wall, there are the challenges of placing that box exactly where it needs to meet the appliance, as well as attaching the appliance to the box.
Otherwise ... as for this particular picture ... I have such a heater, that came factory equipped with a cord and plug. Certainly preferable to having the cable work its' way through a hole next to the box!
Our code - as ridiculously specific as it can sometimes be - relies upon the clearance requirements that are part of the product listing to ban receptacles over such units. I've posted pictures here of installations where there were receptacles in the block walls about a foot above the heaters - while not a good situation, one would have a hard time documenting a violation.
This is certainly an interesting oversight; the code seems to assume that no one would be so silly as to place a receptacle above a heater at all, and allows for receptacles to be built into the heaters themselves.
210.52 had this entry in the online 'draft' of the NEC 2008:
"Permanently installed electric baseboard heaters equipped with factory-installed receptacle outlets or outlets provided as a separate assembly by the manufacturer shall be permitted as the required outlet or outlets for the wall space utilized by such permanently installed heaters. Such receptacle outlets shall not be connected to the heater circuits. FPN: Listed baseboard heaters include instructions that may not permit their installation below receptacle outlets."
I am not sure what 'FPN' means, but I assume it is a footnote of some kind.