Scout often will weaken your deck. Probably not as often as, say coppersmith, but nothing is usually preferable to scout. However, there is something which typically is a good buy which wont run out - silver. Scout gives +1 action, so you can play it for free, but unless you are drawing more than one card, its a crappy cantrip. The deck reordering is ok, but in an engine, theres basically always other pieces to buy. And by the time there isn't its probably time to green. And if you have a bad draw, you probably want silver over scout. In big money, well you don't have very much green until endgame and the reordering is usually useless since you likely wont have card draw with your scout. And silver is even more beneficial in these types of games, so again, you prefer silver to scout.
EDIT: And just to clarify scout vs coppersmith here: if you just randomly stuck a scout in your deck, it will be, on average, preferable to putting a coppersmith in there. But there's bascially never a time where buying scout is the correct strategic choice since there is almost always something better to buy, and even if there isn't buying nothing is usually preferable. Coppersmith on the other hand is on occasion the correct thing to buy, and often can be quite powerful. On average, yeah it's weak, but it's still miles ahead of scout.
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Sometimes you want silver, sometimes it clogs engines. Depends on your deck. Was never arguing scout>CS based on impact. CS can be huge on the right board, but it happens very rarely. Scout is the correct buy more often than CS and some other cards are. Although it's rare when either of them are correct.