The wording of section 76 of the act is "likely to be useful" rather than "which may be of use". I would think "likely" requires the a higher level of proof than most people assume.
Also, to prevent you from taking photographs they need to arrest you - which makes for an interesting situation..... If you are suspected of an offence under section 76 of the terrorism act, surely they should arrest you straight away - to tell you that to continue to take photographs would lead to your arrest is ridiculous - either you have committed an offence by taking just 1 picture, or you have not.
I agree that the issue here is the likelihood that section 76 will be poorly applied rather than it being, in itself bad legislation.
I generally do not have any reason (or desire) to take photographs of members of the military, police constables etc as I am mainly a landscape photographer, however I would NOT stop taking photographs of police officers who were themselves potentially committing an offence because of a threat of arrest under section 76.
I suggest everyone who feels they may find themselves in a position of being asked to stop taking photographs familiarise themselves with the law on stop and search. If you are ever in that situation, knowing your rights and assertively (not aggressively) pressing those rights will normally get you the result you want.
Photographers as a group are becoming paranoid and should chill out a bit.
Having said all this, I am concerned about recent abuses of police power and increasingly worried about our slide towards a police state and the erosion of civil liberties - hell, I'm even seriously considering becoming a member of Liberty !