I do not see this technology as fundamentally problematic (issues of accuracy aside). An investigative technology is problematic, from a civil libertarian point of view at least, if it allows mass scanning of the population at large to detect potentially suspicious activities. This in essence is akin to cops searching people's homes at random to see "what they are up to." However, an investigative technology used to investigate a person already suspected of crime (provided, of course that suspicion is reasonably warranted and not just a mere round up of the 'usual suspects') - it is actually beneficial because it can allow a more accurate determination of guilt or innocence. This is akin to DNA testing.
The P300 technology cannot be used for a mass surveillance - I just do not see how that is possible. It can only be used to investigate people already suspected of criminal activity. So inasmuch as the suspicion is warranted (i.e. there is a bona fide reason to believe that a particular individual was involved), the technology can be quite helpful, provided it is accurate.
Wojtek