For me abet looks very sad on its own, without aid, though far from impossible. (Not nearly as impossible as, for example, the word let [= 'stoppage'] would be on its own, divorced from its partner hindrance in the legal jargon without let or hindrance.)
The four ways in which you can be an accessory to a crime in English law are to aid, abet, counsel or procure it. Accessories do not strictly speaking commit the crime themselves, but are part of a joint enterprise that results in the crime, and are subject to the same penalties as the one who commits the crime.
I think abet still has a separate place as a technical term in English law (and I presume in other common law jurisdictions too). According to the Oxford Reference Concise Dictionary of Law (Second Edition 1990), 'Aiding usually refers to material assistance (e.g. providing the tools for the crime), and abetting to lesser assistance (e.g. acting as a look-out or driving a car to the scene of the the crime)'.