I work in education and the explosion in legal highs, 6/7 years ago, was horrendous. Loads of kids taking all sorts of shit because it wasn't prohibited. We had to do loads of work to explain that legal does not equal safe, we couldn't keep up. The trouble was many kids were already well deep with substance abuse. The damage done is appalling (even that 'spice' crap has killed a few people). Legalising drugs is not some cure-all. I suspect the kids who were doing m-cat every evening will have mental health problems long into the future.
The idea that if booze and fags are legal, then so should skag, coke and mdma is highly flawed. If alcohol and tobacco came on the scene today they would be banned outright. As it is alcohol probably contributed to the birth of civilisation and tobacco-addiction predates modern lawmaking (and prohibition would be a massive headache, education is killing it off in the UK largely). There are loads of skag-heads around here, a real concern for me is getting knocked off of the bike by one, as loads drive cars while nodding (one killed a cycling couple near here a while back).