Does it damage your laptop in any way to leave it on all the time? (Providing there is adequate ventilation)?

Yes, computers in general have mechanical parts inside, as well as electronic components that are directly affected by heat. Leaving your laptop running all the time greatly reduces the life expectancy of it.

Nope, biggest damage to computers is caused by dramatic change in components (unplugging something on a sunny day), theyre made so theyre able to remain on constantly. I work in a data centre, we never turn any of our computers off, some of em have been running 5+ years. Laptops are slightly more prone to overheating so perhaps it's not such a good idea to leave it on constantly, most motherboards turn themselves off before the temperatures get too hot.