The problem is that the message is a "one size fits all" policy that is totally inappropriate at many of the locations where it's constantly repeated. Yes, there is a miniscule chance of a terrorist attack at a major London station but does anyone seriously think that Al Qaeda are going to bomb some unstaffed halt in a remote part of Wales for example?
Also I'm sure I'm not the only one who hears the catchphrase as "see it, say it, sort it", which implies that if you do see something suspicious, you should remove it yourself, which would be more dangerous than ignoring it.
I'm sure these pointless and patronicing announcements are the reason why the majority of us wear headphones when passing through stations, which means we don't get to hear those rare announcements that we actually need to know!