I've never been persuaded by the "gateway drug" argument - as if you're destined to seek more exotic or intense highs once you've tried one type of drug. If it were true then a huge amount of UK university students would end up as heroin addicts or something.
The only part of this "slippery slope" argument I can believe works in favour of legalisation. That there's got to be some amount of "well I've tried weed, it is illegal and my experience proves that what I was taught about it in school was a lie - why not try coke/mdma/ghb/etc..". I don't know how common this line of thinking is but it's certainly something that was on my mind a LOT after I was smoking weed in uni.
