For serious crimes it might well be a stolen vehicle anyway, so the plates won't help no matter how many cameras see them, unless as you say the vehicle used can be identified and tracked fast enough for actual police officers to catch up with it while the perps are still inside.
Meanwhile, the concern with cloned plates is not only the escape of the guilty party but also the innocent person whose plates were cloned, who is probably about to get numerous automated penalties for speeding, not paying congestion charges, parking violations, etc. There is a real problem with the level of such charges and the lack of compensation to the wrongly accused for losses they incur defending themselves. For many people, it simply won't be cost-effective to take time off work and/or retain legal counsel to travel to court and fight the charge, because you'd wind up spending more to clear your name than the original fine on the ticket even if you win.