I must disagree with you on cruisers not mooring in a selfish way, you will quite often see, shall we say a 60Ft space, along come a cruiser and plonks/moors right in the middle of said mooring space. They seem very reluctant to moor to close to any Narrow or Wide-beam.
When a wide-beam moors at an inconvenient spot it IS more noticeable than a Narrowboat mooring at an inconvenient spot, but it still does not make it OK for the narrow boater to moor there. Whilst travelling along the GU I have seen quite a few Narrowboats moored near bridge holes, on bends and opposite trees that overhang the canal from the offside by as much as half the width of the canal, and yes, the odd wide-beam moored inconsiderately.