Slums are good for the economy. The portion of land which is currently a slum is best utilized as slum. Had it not been slum it would have been some piece of land owned by the government which is left uncared and unproductive. If government had some need for the this piece of land then the land would not have been left unoccupied and uncared.
Slum land could have been more productive if it were private property but then if the land had been a private property there wouldn't have been any slum in the first place. So slum lands are more or less unoccupied government land.
Though slums make barren lands productive by giving houses to working masses in city. It creates lots of visual pollution and unhygienic life. For all the other disadvantages of slums its necessary to make sure that there is no barren , unoccupied land in cities where ppl are disparate to settle down.
Slums can be removed from a city just like weeds from a garden. Make the land private property of the slum dwellers or remove the slum dwellers and use the land for construction for the government or sell it. Just as the best way to remove weeds to cultivate thick grass or plants so that there is no clear land for the weeds to come up.
If the land is given to the slum dwellers then they will make as productive as other property surrounding it by construction there or by selling it to people who can make it productive.