Regular maintenance and servicing is also important with a used car because, in most cases, you won't know exactly what the previous owner did with the vehicle. Important scheduled repairs may not have been done when they should have, but if you get your car in front of a qualified mechanic on a regular basis, any problems should become apparent while they are still manageable.
Some people don't worry as much about maintaining used cars because they know they don't plan to resell the vehicle. Still, even if this is the case, that's no reason to expedite its breakdown. There's a difference between keeping your car in prime condition to maximize your profits in the future, and keeping your car functional and safe to drive.
For the sake of safety and your own convenience, it's important to regularly have used car service, from inspections to basic maintenance to repairs. The fact that your car isn't in perfect condition isn't reason to skimp on care - in fact, it's even better reason to invest money in extending the lifespan of the vehicle and keeping it running reliably.