On this 3 mile stretch, it's impossible to overtake. And invariably some sad loser* will stick to 20, so everyone has to.
Same as on the roads with 30mph average speed cameras - the cars obeying the limit are a natural brake on the fucking idiots (which is what my spellchecker changes "selfish cunts who speed" to).
The problem with whining about 20mph speed limits is that even on a 5 mile uninterrupted journey, you only save 5 minutes. Which you would lose in a single set of lights in the centre of Brum.
Also 20mph is much kinder to the roads, the atmosphere and - assuming you aren't a thickie - your pocket. (Accelerating to 30mph requires 2.25 times more energy than accelerating to 20mph. Driving at a steady 20mph can reduce urban fuel costs by up to 30%)
Hope I haven't offended any thickies out there.
(Also, it's a law of nature that for every racetrack that has been saddled with a 20mph limit, there will be a single track switchback country lane somewhere that has a NSL posted.