There are a number of factors which dictate the speed of cards required for digital cameras. I will list those that I can think of below.
i). The size of image files - If you shoot small jpeg, there is a lot less writing to be done than if you shoot Large RAW + Large JPEG. Some DLSR's require 40-50MB's per shot for the latter, so if you have a Class 4 SD card (min 4MB/s) it can take ~10 seconds to write each shot.
ii). Buffer size - Most DSLR have image buffers that can store between 6 to 50+ shots before slowing down. My 7D would buffer about 20 Large images before slowdown, my 6D stores 11-12 with a slow card, or 16 with a fast card.
iii). Write time after buffer full - A large buffer is nice and can to hide some of the limiatations of slows card. However, once that buffer is full a slow card will really bog down any DSLR. My 6D for example slows to one frame every 3 seconds with a Sandisk Class 6 card which writes at 8MB/s, but can hold 2fps with a Sandisk Extreme rated 45MB/s. 2fps is still usable, 0.3fps is not.
iv). Compact cameras - These tend to have much smaller buffers and can suffer massive system slowdowns whilst writing to SD. I own a Samsung NX1000 which is a great camera, but menus almost completely freeze whilst the card is being written to. Fast cards are necessary for this compact because the buffer is small and the processing power is low. DSLR's tend to be better.
v). How you use the camera - If you only shoot landscapes you do not need the fastes card. If however you shoot sports or regularly use the high-fps mode, faster is better.
In summary, you can never have too much speed. Ideally your card should be atleast as fast as you cameras internal write speeds. Older DLSR's may only write at class 10 speeds (10MB's), so it could be a waste of money purchasing the fastest cards, but atleast you will know it is performing at it's maximun. My 6D seems to have an internal write limitation of 45-50MB's, so it is not really worth me spending £60 for a Sandisk Extreme Pro card (90MB's) when a £25 Sandisk Exterem (45MB/s) will work 99% as well.
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