Scout is not the worst card in Dominion... the worst card is Curse. I´d rather like to get a scout ambassadored then a curse.
But Curse only costs $0, while Scout costs $4, and you can even gain it for free in tricking your opponents to play Witches...
I think most things have been said, but anyway:
Of course Scout is not completely useless, and of course you can lose a game by ignoring it because it is a 'bad' card. In a game, you should never judge a card by if it's strong or weak on average, but if it fits your strategy.
Scout can clean up Apothecary and Scrying Pools, but as said, the main weakness (in contrast to Duchess) is that it usually takes a buy and $4 to be gained, and the situations where it's better than having nothing (let alone something else for $4) in the deck are very rare. They exists, and situations where you have a spare $4gain also exists, but they are rare.
In contrast, Workshop gives you exactly this $4gain, which can be very very welcome in building engines, especially if you are on low economy thanks to strong trashing.
And Duchess does not have the problem of costing coins and a buy/gain, but they might come for free with Duchies. Especially in slogs you have that Duchies are important and the filtering might be helpfull to you (maybe more than to your opponent if they don't mirror you), and an additional terminal Silver in the deck is welcome. So I see a clear spot for Duchess in the game, in addition to all the niche usages one can think of (need terminal Silver for KC and nothing else on the deck, playing Scrying Pool with Fishing Villages and need terminal Silver, etc.pp.).
Scout in contrast only has these niche usages. Of course, if they appear, one should use it, but every scenario that seems generic for Scout (Alt-VP, XR) turns out to be usually better of without him.