Here is what the opposers will say to your answers, GHoST (nothing personal)

ITEM 1:
And what of the rights and freedoms of those that choose not to use cannabis?
Cannabis use is not victimless - how many families have split due to one (or other's) using cannabis - lethargy, non-compus-mentus, etc?
The 'war on drugs' is largely funded by monies that have been seized from drug related activities, the extra burden to the economy can be off-set by the reduction of drug-related illnesses that put a constant strain upon the national health service.

ITEM 2:
If an accident occurs at the work-place, and it is found that the person responsible for that accident is under the influence of cannabis...well, you can guess the rest lol
Alcohol inebriation is a great deal easier to detect than cannabis (fumes on the breath alone), and is easier to quantify (alcohol/blood levels).

ITEM 3:
How would you quantify the level of cannabis exposure, and furthermore a persons judgement skills thereof?
Is cannabis detectable? And to what degree? (<--I genuinely don't know the answer)

ITEM 4:
You'll probably find that the drugs test that police can administer only detect harder drugs (coke, heroin, etc)...certain over-the-counter medication will show a positive result for cannabis (Ibruprofen, for example). Moreover, what powers of arrest would they have with cannabis? You can sit in a room of tokers, and still test positive.
There is also the consideration of company insurance, vehicle insurance, limitations of use acts, etc, etc.

ITEM 5:
One man could make an ounce last a month, other people would see an ounce as being a sessions worth, or daily supply....What if you are using it for mecidinal purposes? An ounce might only be a days worth, init!


See, what seems perfectly reasonable to you, is most likely totally absurd to another.
There has to be compromise across the board.

How does Amsterdam do it?
Contrary to what most people think, Amsterdam still has a drug problem...mostly from visiting tourists, it has to be said lol
There's THAT to consider, also!

The UK is already a very attractive destination because of our stupid benefits schemes...add free-reign of drugs to that, and all of a sudden.....we got big problems