Let's not obfuscate and/or split hairs. Your point was that speed cameras are no good at all because they only catch some offenders. Tony's point was that this is just fine - no one strategy fixes all ills.
OK, fair enough - we got into a bit of a cul-de-sac there.
Right, so to address the above point. I think you/Tony are over-simplifying the argument I think I put forth. The problem is that when they "only catch SOME offenders", the sampling is not random, and is in fact seen as highly "unfair" (that's a great term, isn't it?!?).
I shall paint a school-days analogy, if you'll forgive me.
Imagine you, a lifelong goody-goody, and some "known troublemakers" are all caught smoking behind the bike shed. You hang your head in shame, whilst the others leg it, taunting the teacher as they go, in full view of your peers. You find yourself the only one outside the head's office at going home time. No child psychologist is required to guess that you feel hard done by. Some might say that the school's justice system has taken a knock due to the visible failure to bring the others to justice.
Now, I'm not saying that you should have got away with smoking BECAUSE the others escaped, I'm saying there are subtleties at work here that need considering.
To be honest, if you still don't see this is a valid viewpoint, I shall give up, no hard feelings. I think speed cameras have good and bad points, most of which have now been discussed here or on other threads, and they are difficult to evaluate. My view, currently, is that the way they are currently used does as much harm as good. I do think speeding needs to be tackled in the UK today, but so do other big safety issues.
Your view may well differ to mine!