There are a couple of reasons to slow down, one being comfort, the other avoiding accelerated suspension wear/stress. If you mean 'why should speedhumps be there in the first place', that's a whole new debate. I think they are warranted in some places, but yes pointless in others, although I can honestly say that I've never been massively inconvenienced by having to slow down a few times for them.
I wouldn't worry about upsetting following drivers by the way ScoobyChris, I can't imagine their mood is that important to your task of driving..
How fast previous cars could clear speedhumps is a nonsense argument to be honest, I would imagine that a Range Rover could get across them at considerable speed without any discomfort, but then it is an entirely different sort of vehicle isn't it? Given the set-up of the 100hp, is it any surprise that it doesn't float over speedhumps as though they were pillows? As such, unless your entire daily commute is made up of speedhumps (and if it is, you bought the wrong car), they really shouldn't be a concern when talking about a cars ride.
I came from a VX220 to the 100hp, and can say in all honesty that it is not a hard riding car, it's just not by any means a limousine either.