Speaking for western medicine, I will say that the only demonstrated benefits to THC (in any form) are to decrease nausea and stimulate the appetite.
Pill form (marinol) is the best method of adminstering cannabinoids. Smoked is the absolute worst, since there is absolutely no way to control the dose. And of course, smoked has many other negative effects - 1 joint has as many carcinogens as 5 tobacco cigarettes, as one example. But many people prefer smoked because that is the only way they get the buzz.
Personally, I don't care if people want to smoke pot. But I do wish they'd stop pretending it's medicinal.
While decreasing nausea and stimulating the appetite are particularly important for many cancer patients (usually those undergoing drastic chemotherapy which tends to have very negative effects on both of these), there is also a large benefit for cannabis in chronic pain conditions including rheumatoid or osteoarthritis and neuropathic pain.
At least one person I know has responded very well to marijuana treatment who would otherwise require a constant morphine drip to control her pain. She has been to just about every specialist in Southern California, and tried marijuana as a last resort.
Really? Pill form is the only way people can get a buzz? So the brownies, cookies, candies, and other baked goods, as well as the alcoholic extracts are ineffective then? (Um, no, actually, though it is true that the absorption rates will differ dramatically depending on the form of the dose.)
As to the original poster's question about kidney damage (from Wikipedia)
THC is metabolized mainly to 11-OH-THC (11-hydroxy-THC) by the human body. This metabolite is still psychoactive and is further oxidized to 11-Nor-9-carboxy-THC (THC-COOH). In humans and animals, more than 100 metabolites could be identified, but 11-OH-THC and THC-COOH are the dominating metabolites. Metabolism occurs mainly in the liver by cytochrome P450 enzymes CYP2C9, CYP2C19, and CYP3A4. More than 55% of THC is excreted in the feces and ~20% in the urine. The main metabolite in urine is the ester of glucuronic acid and THC-COOH and free THC-COOH. In the feces, mainly 11-OH-THC was detected.[69]
So for this drug, the liver is the main route of elimination, with a minor component from the kidneys as well. Damage and toxicity tend to occur when there is accumulation of the drug over time in the liver or kidneys; this does not appear to happen with THC.
One estimate of THC's LD50 for humans indicates that about 1500 pounds (680 kilograms) of cannabis would have to be smoked within 14 minutes.[24] This estimate is supported by studies which indicate that the effective dose of THC is at least 1000 times lower than the estimated lethal dose (a "therapeutic ratio" of 1000:1). This is much higher than alcohol (therapeutic ratio 10:1), cocaine (15:1), or heroin (6:1).
I'm still not sure how the 1500 pounds number was arrived at, since that would presume (using the math above), that a person smoked 1.5 pounds normally... but it's a pretty huge amount required to have an acute lethal dose regardless.
Chronically, I don't know of any study that has found evidence of kidney (or liver) damage in regular consumers of cannabis, but as with anything, if you take enough of it over a long enough period of time, it's probably possible. Water is certainly fatal in large enough doses.