Methinks that the original reason private for 'letting' cyclists into bus lanes - long before sustainability was fashionable - was because it was bonkers to have us on the outside edge of a bus lane 'getting in the way' of cars.
Using the same logic that it would (puts on flac jacket) make sense to permit motorcycles *that are limited to 30mph* into bus lanes on roads *where the limit is greater than this*. I regularly see milk floats using the local bus lane (Hinckley Road, Leicester) completely unlawfully, but it makes perfect sense.
Where this thread seems to be heading, it seems to me, is to argue more in favour of bicycles and HOV (High Occupancy Vehicle) lanes - that's to say, lets allow anything, taxi, minicab, bus, and, eeek

, even private cars into bus lanes when they are carrying more than just the driver. I don't see the logic of allowing taxis into bus lanes when they are 'running empty' (no, I don't beleive they are all going to the next job).
However for HOV lanes to work, they need to be properly designed, and driver behaviour needs vastly improve over the current parlous standard of most driving that one sees!