Just like anything else, there are pros and cons about things. Ideally the phono cable should have no connections from the headshell all the way to the phono preamp. However, this limits you to either the factory cable or one that is a continuous run aftermarket cable, of which there are few. The factory cable on a Graham is probably a good one, but the Robin is their lowest priced arm, so there will be some corner-cutting.
The dealer is covering his bases, because by recommending a cable upgrade, he can make some money, and he can tell you that "he tried to tell you" if you come back later looking for an upgraded cable.
That said, at the Robin's price point, you probably could benefit from a cable upgrade, but it would be wise to research it yourself first, instead of making a spur-of-the-moment decision.
The dealer is covering his bases, because by recommending a cable upgrade, he can make some money, and he can tell you that "he tried to tell you" if you come back later looking for an upgraded cable.
That said, at the Robin's price point, you probably could benefit from a cable upgrade, but it would be wise to research it yourself first, instead of making a spur-of-the-moment decision.