There are a number of arguments for the legalisation (or, rather more accurately, the decriminalisation) of cannabis, and I'm sure you've heard most of them before.
The most obvious - and most oft used - is that cannabis is no more harmful to you than smoking too many cigarettes, having 10 espressos in a row or having ten pints in the Hogshead on a Friday night. The counterargument is that, in fact, there is evidence that it does nasty things to you: smoke it neat (i.e. without tobacco), or smoke a particularly well refined bit of the dark stuff and you might hallucinate, or it can lead to short term amnesia, buggers up your brain cells and so on. But then is it worse - we know it has an effect - than the other so far legal drugs which we take with barely a thought for the effects?
Other arguments (and to my mind far more powerful ones) include:
(1) Laws are effectively social engineering; laws are passed because we, as a society, have determined that a certain course of action is detrimental to society as a whole, and therefore needs to be legislated against. Cannabis use falls under this heading, presumably. But because laws are effectively social engineering they are only really enforceable if society - as a whole - agrees with them. It appears that this day and age very few people do in fact agree that cannabis should be an illegal substance. The police will very often let you off with a good telling off if cannabis is found on you, at worst they will issue a caution. Rarely, if ever, will you be prosecuted and those rare occasions which have led to prosecutions have often failed (in respect of pure possession, at least) because juries have refused to convict. There is also the argument that to force prosecutions to go ahead will lead to a whole section of society (a particularly large one) suddenly receiving criminal records where previously they were perfectly law abiding.
(2) It is said that cannabis use leads onto more serious, harder drug use. So far, there is not one hard shred of evidence that that is in fact the case, but - for the sake of argument - let us assume that it does. Predominantly, this is because as things stand in order to obtain cannabis you come into contact with fairly shady characters. You feel like you're doing something dangerous, something slightly naughty, and you're psychologically more prepared than you otherwise would be to try other things. Those who smoke cigarettes - highly addictive - do not necessarily immediately go off and buy a kilo of heroin. Why not? Those who smoke cannabis - not at all addictive - according to this theory do. Why? Remove the element of illicitness, start charging tax on a packet of 20 Cannabis, and you would find that there is no longer such an obvious progression. In fact you would find the opposite; at present cannabis is an illicit substance, a banned drug. So is cocaine, so is heroin. All one big happy family. Cigarettes - which to my mind are far worse for you - are perfectly OK, therefore one does not lead to the other. Decriminalise cannabis so that it is no longer associated with hard drugs and you would find - as with cigarettes - that there is no longer such an obvious connection between the two.
My point is simply that cannabis is
1. no worse for you than other, presently legal and accepted drugs;
2. was made illegal simply because it was found later than tobacco (ever asked yourself what would have happened had cannabis been found first and tobacco second?);
3. is, whether you like it or not, actually accepted by society, and no longer capable of control by criminal enforcement;
4. has never been proved to have led either to serious injury, death or criminal activities.
Why is it banned?