Scout is a fine card when you pay effectively $0 for it. It smooths out your green draw and can allow you to better setup engines. Basically playing a scout decreases your hand by 1, but increases your search space for lining up components by 3.
For the price, it is rarely worth it (Mystic and especially Herald can do a lot better with Scout), but when I'm grabbing it for close to free - off an Iw or something - I will grab it for a number of engines if there are no longer useful components.
For instance, say I have something like Village/Iw/Hunting grounds. Village alone allows you to search 5 cards for a live Hunting grounds. Scout lets you search 7. That is a pretty big increase in the odds your engine will hit, pulling green out of deck is just gravy on top. When card draw is cheap, Scout gets better, the problem is that when card draw is cheap you normally want more of the draw instead. But if Scout uses otherwise useless gains (e.g. Iw), then it tends to do a decent job smoothing out engines. It is only when the opportunity cost for Scout falls that you really want to gain it for reliability purposes.
Similarly, Pool/Scout/Gainer is a decent step up from Pool/Gainer. Yeah grab your stack of Conspirators or whatever first, but if you have a Workshop, Pool gets a lot more reliable as Scout pulls out the odd stop card and can position your deck to discard a treasure stop. Given that Scout has a low effective card size with Pool (maybe something like .5 or .75 cards), it isn't a bad gain late-ish game.
The big question is how much extra support you are willing to allow on a combo. Scout/Xroads is borderline at best if the Scouts are free, but it gets a lot better with cards like Tr or Prssn so you can play a lot more Xroads. Likewise, Scout/Tunnel tricks are not too impressive, but if you get a nice constellation like Scout/Cellar/Apprentice/+buy it can be decent at juicing the Cellar (and worst case, you Apprentice the Scout). Most cards have good setups when you start talking 3 & 4 card comboing, but that comes at the price of being really rare in random play.