Hi Maarten,
Some notes I made a while back on the problems with including Grade II (and equivalent):
a) The lists become huge and difficult to maintain;
b) Grade II contains many of the "non-building" listings; looking at the 703 Grade II in Cambridge, the first few on the list include a statue, a boathouse, a couple of walls, and some gates. On my way to work, I cycle past two telephone boxes, an unremarkable river lock, some respectable terraced cottages... all grade II listed, and all within a few hundred meters on the same road. They're individually registered, but they're not what the general public would consider "listed buildings" (and personally I'd argue that a lot simply aren't very interesting from Commons' perspective)
c) Because it contains so many buildings, a lot of them are still people's homes. This gets us into some complicated and concerning issues; I am not sure there's much benefit to running a contest encouraging people to go and take lots of pictures of relatively anonymous houses, and it's quite possibly going to backfire if we do. It's still something of an issue with WLM in general, but I'd argue that a Grade I or II* building is less likely to feel weird in this regard - if you live in it, you're a bit more used to the idea of public attention than if you live in a less distinctive building. Grade II almost certainly contains a much higher number of private residences both proportionately and absolutely.