Not that I don't love slum lords...It's because of them I've found some of my best deals! Namely power of sales and huge discounts due to huge problems with both the tenants and the property.
Milking a property by accepting the income but not putting anything back in is not only bad for business, but gives all landlords a bad name.
When you wan't to sell, (or are forced to!) you will have to sell at a discount due to the properties problems, that is if you can sell at all!
When I say I buy crappy fixer rentals, I don't mean they stay that way. We fix the issues (tenant and building) and can get decent rents because we make it a decent place to live. We usually figure on 10-20% of purchase for fix-up costs.
You can't get something for nothing...if you are going to accept the rents, accept the responsibilities as well.