Bertrand Russell, of course, is a master of logic. But if you are drawing from his History of Philosophy, I'm not sure that is a particularly good source for views on ancient philosophy. I tend to think the best way to justify a claim is to look at the primary sources themselves (if we have them—which in the case of Plato we are pretty sure we have everything he wrote and then some extras).
In terms of scratching the surface, much of what Plato contributes is ways to approach particular problems. For example, if you look at the treatment of universals in the Parmenides and the Philebus, the approaches in the arguments here (severally considered) have a lot of staying power: in particular, you find related arguments being made regarding universals by Aristotle and on, going all the way up to the present day (including, for example, Bertrand Russell). What is going on here is not merely philosophical scepticism (though there is that), but also approaching and techniques that need to be solved in order to make progress on a particular problem. So, that is an example of how the dialogues go beyond simple scepticism. There are so many other examples: the dialogues are packed with them — which is part of the reason why treatments of many modern philosophical problems and approaches can be traced back to Plato.