Too much backpressure is harmful, yes, but some backpressure is helpful. I know that in the oil and gas industry, pumps often have backpressure to assist them in keeping air pockets from developing (causing cavitation).

What engines need is low backpressure, but high exhaust stream velocity. A fast-moving but free-flowing gas column in the exhaust helps create a rarefaction or a negative pressure wave behind the exhaust valve as it opens. This vacuum helps scavenge the cylinder of exhaust gas faster and more thoroughly with less pumping losses. An exhaust pipe that is too big in diameter has low backpressure but lower velocity. The low velocity reduces the effectiveness of this scavenging effect, which has the greatest impact on low-end torque.

Low backpressure and high exhaust stream velocity can be achieved by running straight-through free-flowing mufflers and small pipe diameters. The only two exceptions to this are turbocharged engines and engines optimized for large amounts of nitrous oxide. Both of these devices vastly increase the exhaust gas volume and simply need larger pipes to get rid of it all.

Getting zero backpressure (ie, no restriction) is, to my knowledge, impossible with muffler and cat. The designs of the muffler pretty much always include multiple bends and/or baffles, all of which decrease flow rates and can result in backpressure. I don't know of any street-legal system that achieves it (but would love to know if there are any).

As far as mufflers go, I'll explain a bit more.

To get the quietness most people want, a muffler must have an intricate, labyrinthine flow path to help slow and cool the hot, vibrating exhaust gas. It contains baffles that cause the exhaust flow to reverse direction and intermix. These are great for reducing noise but are not so great for flow. The twists and turns the exhaust must endure in a stock muffler are restrictions that cause excess backpressure. You can run in a straight line faster than you can run in a tight, fun-house maze, right? The same goes for your exhaust gas.

To produce the most power, an exhaust should have minimal restriction on the exhaust flow. Restriction hampers the burned exhaust gases from exiting your engine, causing some charge dilution with the incoming fresh fuel air mixture. If all the exhaust gas cannot escape from your cylinders, it dilutes the flammable power-producing intake mixture that is trying to come in. The diluted mixture does not burn as well as a pure mixture. This causes a loss of power. You don't feel so energetic at a packed club with lots of cigarette smoke, sweaty bodies and hot stuffy air right? Neither does your engine.

With greater restriction, backpressure is generated, making the engine work harder to pump the exhaust out of the cylinders. That work could be used to turn the wheels instead. So, in the case of the exhaust, backpressure is not good. Read back to see how backpressure does aid the motor at times and various situations.

In essence, a low backpressure system is often only available with low velocity exhaust systems. This lower velocity reduces the effect described above, reducing torque. Thus, it's not so much backpressure that's to blame, but the system that usually causes low backpressure (big exhausts with slow-moving exhaust gases).