In addition to what Larry says, when you change your oil from the bottom (without a TopSider) you have the added benefit of being able to inspect the rest of the car from below. There are so many components under the car that, if left uninspected, can go "wrong", and this "preventive maintenance" can avoid future problems. Fo instance, you can inspect the fuel pump, accumulator, and hoses for possible leakage or weepage (just check all the previous posts which found this to be the cause for hard starting), or you can check you exhaust system for rusting (which could cause your asphixiation if it's leaking), or you can check for curb damage to suspension components (and just check all the previous posts of people experiencing suspension problems); I thknk you now get the idea of this kind of "preventive maintaenance".
I will submit again, at the risk of incurring all the wrath of TopSider fans, that using the TopSider is not in the best insterests of your car, but rather in the the best interest of your convenience. If you are afraid of getting your hands oily, wear latex gloves; and if you are afraid of getting on your back under the car, use a cardboard sheet, and don't ever go to bed (you'll be on your back there too).
In conclusion, the use of a TopSider is for people who THINK it is more convenient (check Aaron's posting that it take 5 minutes of pumping, whereas doing it from the bottom takes 30 seconds) and goodness knows I agree with Aaron most of the time. The TopSider is for people who don't care about the rest of their preventive maintenance, and think that doing an oil change in a tux is acceptable behavior.
How's that for an opinionated posting?