I always imagined that spirits were like a highly dense liquid held together by the spirit's psy defence. Physical attacks would not cause real damage to a spirit's liquid body, but it would disrupt the spirit's ability to hold itself together.
I like this theory, that a spirit is held together by a mind shield. As we know, a mind shield can be affected by both physical and psy attacks. And when the mind shield breaks, the spirit dissipates. Perhaps, though, the theory doesn't require that the spirit be composed of a dense liquid; it doesn't matter what it's made of (whether it's liquid, pure psy, or something else) as long as the mind shield surrounding it is tangible. As for the weapon, if we assume it's also ethereal, perhaps the mind shield around it is harder? Set is able to create a highly durable solid state shield, right?
The spirit being affected by physical attacks could be because of latent psy talent; I suspect that most people within the telepath world have psy power, of varying degrees, which probably goes unnoticed. But, because the person intends to attack the spirit, and believe they can harm the spirit, they focus their latent psy power subconciously, and do attack the spirit with their mind, without realising it is a mental attack. Because it isn't a proper psy attack, it's not as effective as one, resulting in the 'physical resistance' possessed by spirits.
My problem with this explanation is that it assumes that the 50% reduction of physical damage is a uniform property of all potential attackers, whereas it being a property of the spirits themselves is far more likely. As you said, people in the telepath world have psy powers of varying degree. Surely Duvalier, a talented psy, would (subconsciously or not) deal a higher percentage of damage with a knife than Qudssi, a man with little to no psy abilities?
Allow me to suggests another theory. This theory assumes that the spirit is merely an intangible disembodied mind of a deceased human. Spirits are entities which cling to their former lives, so much of the habits and brain functions from their lives may remain. When a spirit is attacked by a physical weapon, its mind still
thinks it's being harmed, so it may shut down or die from shock. Pulling a quote from the Matrix, "your mind makes it real". It's somewhat like a placebo. Of course, this is less effective than actually being wounded by a weapon, so only half damage is taken. Greater ghosts, which have been spirits for a long time, may have forgotten much of their humanity, no longer associating being struck with something harmful, thus making them immune. (This may also be why the heads of greater ghosts are skulls: they have forgotten their humanity and now perceive themselves as such.)
Also note that for this theory to work, their weapons would have to be real, and wielded with telekinesis.