The cat could be omitted if the length of the exhaust piping were properly tuned for the desired load range and emissions wasn't a factor. Not really an option on a street car. The cost of tuning an exhaust alone is extremely high. There's no system that's perfect for all load ranges either. You almost always have to make sacrifices.
In a system with noise control you generally want to stick with muffling devices that use frequency cancellation technology, not something packed with baffles or perforated tubing and fiberglass. The exhaust oscillates by nature, there are devices which generally serve to fill in the gaps in air flow using harmonics. This grants a much more turbulent free exhaust flow, something which even simple perforated tubing doesn't do.
And before anyone asks, no, running an open header is NOT the best solution, you lose the potential gains you would otherwise see from a system properly designed to take advantage of scavenging.
All of this is true for turbocharged systems as well, the thing to remember there is that when they're boosting they displace MUCH more air than a N/A. The overly generalized saying that "you can't go big enough" with regard to a turbo equipped car's exhaust is [strike]simple ignorance[/strike] not always the case. It depends entirely on the power goals, the amount of air displaced, the desired RPM range for peak efficiency and so on. EDIT: It may be simple overkill or it may be enough effect your vehicle's efficiency when you're not boosting... I know, something few people really care about.
EDIT: Oh, and oscillations in the exhaust flow are not a bad thing across the board. There are ways of tuning an exhaust to make use of the sudden increase in suction.
