It'll do nothing to stop irresponsible dog owners being irresponsible.
It's useful for responsible dog owners as above, if it gets lost/stolen it can easily be identified.
As someone above mentioned, mobile phone use in cars is rife, and whilst fineable, not really enforceable, like middle lane hogging.
How do they plan to enforce it, issue all plods with wands so they can swipe any dogs they come across?