Tacho does key coding milage and scans of fault codes etc too, but i only use for hex dumps, skc retrieval, and milage correction, usually commander works where vag tacho doesnt.
they probably do a lot more too, i know the genuine can-commander could be really useful, for instance if you have a fried ecu and replace with a doner cars ecu without knowing the skc. I tried to help a guy on this forum in a situation like this a while back with a bodged chipped ecu. Well vag commander would be able to marry that doner ecu to the car clocks/keys etc. Currently i think some people offer a postal service to get this kind of replacement working, maybe with immobiliser deletes etc which of course would invalidate insurance.
As with everything, dont mess with things you dont understand, VCDS is very user freindly and has good tech support, if you know what your doing i guess a copy cable could suffice. Id be interested to see what these good copy cables can and cant do, and what quality the internal components are. Nothing could be worse than corrupting an eprom write with a short to ground through bad components!