Can an Alternator Cause Components to Run at a Higher-than-rated Voltage?

As Kevin said, anything intended for automotive use will have this taken into account. These systems may be commonly described as "12V" to the public, but it's well known the voltage will be higher.Designing electronics for automotive "12V" power can be a challenge. The fact that the battery charging voltage, and therefore the running voltage most of the time will be up to nearly 15V is no big deal. The electronics does have to be designed for a wide enough average voltage range, like 9-15 Volts if you want it to operate over most conditions. The much tougher part is that spikes of 10s of volts can happen regularly. For example, you might be tempted to use a ordinary 7805 regulator to make a low current 5V supply from automotive 12V power, but it might not last very long. There are special regulators specifically designed to tolerate the high voltage spikes.Another issue is that even the 14.3V you measure may only be a average over 100s of ms. Old fashioned alternator regulators were just a thermal voltage switch that turned parts or sometimes all of the field windings on/off. They might only vibrate a few times per second. You got average 14.3V, but the peak to peak variations could be substantial. Modern regulators switch faster and react faster, so this is less of a problem with newer vehicles.Yet another issues is the extreme temmperature range. The same circuit has to work in Phoenix after being parked in the sun for 8 hours and northern Minnesota when starting on a winter morning.

As for the ordinary LEBs (light emitting bulbs) you ask about, this doesn't shorten their life because they were designed for this environment and the quoted lifetime already takes this into account. You should get the life it says on the package. However, you should get substantially longer life from a car headlamp if you were to only run it in a lab with a well regulated 12V power supply. Of course that's not much use in lighting up the road in front of where you are driving

Given that the voltage regulator on my motorbike is set to 14.3 volts (so that it charges the 12 volt battery effectively) and given that the alternator is what provides power for the electrical system while the battery is charging, does this mean that the components in the system rated at 12 volts are being run above spec (ie. at 14.3v)?

Is this safe or could it shorten the life of the components (which are mainly regular incandescent bulbs and LED bulbs)?

Part of me thinks that the designers are relying on the voltage drop caused by the long runs of cable in the wiring loom to effectively reduce the voltage to near 12v by the time it reaches any components.