I'll partially disagree with the part about engine flushes. An engine flush is usually recommended whenever the oil hasn't been changed on time based on mileage OR time. I would recommend that as long as the oil changes are normally regular and the most recent oil change was overdue. Also, about every 15,000 miles (or every FIVE oil changes).
I'll agree not to use it in the case that the oil change history is unknown, and/or the engine has a lot of miles on it. Once regular oil changes are performed on the vehicle again (about five without going overdue), and you have verified that there is little to no oil consumption, an engine flush is perfectly fine to perform. An engine that is cleaner on the INSIDE will pollute less on the OUTSIDE. In most cases, if its cleaner on the inside, it also runs more efficiently. For the higher mileage engines, maybe use an oil that will also recondition seals and gaskets and prevent/reduce leaks, burning, and consumption.