Thing is, there are more efficient methods of reducing crime. ASBOs tackle the symptoms, but what you need to do is tackle the causes. One way of doing this comes out of the "Broken Window" theory - that broken windows lead people to think that anything goes. By clamping down on minor crimes, major crimes drop hugely. All this without having to criminalise behaviour that is not criminal, but annoying.
Alternatively, how about tackling the root issues that cause crime: social deprivation, poor access to education, low employment, and so on? Obviously, I'm not going to propose a solution to how the British government can do this - I'm not a sociologist or a social worker who can plan all this out. I do know it's possible, though.
Also, any ASBO that prohibits someone from hanging around with other teens is an abuse of human rights. Period. Even if they were shown to be 100% effective, I'd be against them for this reason. As for collateral damage, I can't agree with that either. Better that 100 guilty men walk free than an innocent man hang, as the saying goes. People are people, even when they're making people suffer. You can't throw away the rights of a few who aren't doing anything illegal to safeguard the pleasure of the many.
As for legitimate motives for these laws, might I suggest the desire to appear tough on crime? In the past few elections, the major parties in the UK (and in Ireland too) have been going on about how they are tough on crime, how their actions will make crime less bad and the other parties' will make it worse. Criminalising this kind of thing is a great way of seeming tough on crime without having to invest much time, money, or thought.