The most important points that could be used against legalizing drugs are:
We do not want people to use drugs at all. If we legalize drugs, we are sending the message that using drugs is, in some way, acceptable. This will lead more people to use drugs.
More drug use will lead to increases in crime. Even if we decriminalize the use of drugs, there will still be many drug-related crimes. If we encourage more people to use drugs, we will see more people commit crimes while on drugs. We will also see more crime as addicts steal to get money to buy the drugs they need.
More drug use will lead to other negative social impacts. There will be a greater need to pay for health care for drug users. More people will become addicted to drugs and will become much less useful to society as their ability to work decreases. More drug use will lead to more domestic violence and to more families living in poverty.