What sedanman said. There is just one fuel nozzle into the throat and the barrel valve carb simply provides better fuel metering throughout the range. Makes it easier for engine makers to tweak everything from performance to emissions.
As far as replacing them it depends on a gut feeling when doing inspection. If the purge system is suspect I replace the carb. The check valves in these things tend to stick due to bad fuel and even good but reformulated fuels. Most are replaceable only by ordering the pump body and if you have the cost of a repair kit, pump body and your labor you're about even with just installing a complete new carb. Zama's are more suspect of needing replacement then Walbro in my experiences.
The check valve deal is actually true for the butterfly/cube carb but the barrel carbs tend to cost less for a new one then many of the cube carbs. Depends a lot on just that, replacement cost versus correct repair cost.
