;-)
I travel a few 1000 kilometers/year with one of my MBPs (including the charger). No stability problems.
Cheap, compared to other options. I think external power supplies are not the problem.
Often, a MacBook (Pro) charger will do just fine for the whole Mac life. However, it was actually disturbingly frequent that, on previous generation chargers with the "sugar cube" Magsafe plug, the low voltage cable had a strong tendancy to wear itself to the point of shortcircuit inside the insulation, which was not covered by warranty. Even the "aluminum tube" style of Magsafe plug isn't entirely free from this problem, even though the low voltage cable is sturdier.
White MacBook: charger cord broken in the plug itself. Unrepairable.
Previous unibody MacBook: charger replaced twice under warranty for failure in the cable. Mac sold after AppleCare expired.
Current MacBook Pro: charger low voltage cord failed close to the brick. Warranty denied. Crack opened brick, used electrical domino to join wires back. Too cramped to actually solder it back. Strain relief gone, brick held together with electrical tape. Too fragile leave home now, as water from dripping boots would easily get inside when laid on the floor of coffee shops in winter. Around the same time, I accidentaly let the long cord make its way in a turned on radiator. Cord fried in a big spark.
A $100 charger is very cheap compared to a $3000 machine, sure. But compared to a $500 used MacBook, that's another story.
My suspicion: laptops and chargers from people who haul them on foot all day long take much more of a beating than professional laptops moved from time to time in a motor vehicle.