Expand is not as bad as Scout. I guess the biggest thing I've learned about it here is that its strongest use case is probably as a terminal Gold +buy when nothing else is available.
It seems like almost every time one would get an Expand, you'd be at least as happy picking up a Graverobber, Rebuild, Remodel, Wharf, or just a Gold - but only the last of these is always on the board.
It isn't that great at anything that it does. It's just an overpriced substitute for a lot of different things on most boards. So I overstated its badness, but "anything you can do we can do better" to me seems to describe most Expand play in practice.
Well, anything that Jack of all Trades does, Squire, Spy, Watchtower and Chapel can do better. JoaT isn't good because it gains silver, or because it self-Spies, or because it draws up to 5, or because it trashes a non-Treasure card, but because it does all of them. Similarly, Expand is good because it's much more versatile than any of the large number of cards that do something better that Expand does.
Jack is good because it does all of those things in the same turn. Getting a Silver AND trashing an Estate/Curse AND self-spying AND drawing a card is a lot better than the case where you have to pick just one. Jack wouldn't be that great if you had to pick just one every time - actually, it would suck pretty hard. Expand makes you pick just one. If Expand let you trash 2-3 cards and gain one costing up to $3 more for each trashed, then that would be a whole other story...
Then compare it to Steward or Count.
Steward and Count don't whiff nearly as often, and aren't overpriced for what they do. And Count is a lot more flexible than Expand - arguably, so is Steward.
(
Edit: Steward is sort of overpriced for doing just one thing - Chapel, Duchess, and Moat cost $2. Note that Chapel is "underpriced" at $2, or at least "the best card in the game", and you often only need to trash two cards in a turn anyway - in other words, Steward is a great first-shuffle buy that turns into +2 cards or a terminal Silver when its job is done. Not only is Steward is super-flexible, but the difference between costs of $2 and $3 is minuscule compared to the $5-$7 difference, and Steward is much more flexible than Expand. Except for when the junkers are just that strong and you have 3-4 players, you're going to draw $3 and one buy much more often than $2 and one buy anyway, and you wouldn't trypically spend an extra buy on Steward, either, because it's kind of late by then.)
Anyway, I gave Expand a little bit of credit, and I still think it's mostly bad, but it can do a job sub-optimally in a pinch, but... that's about it.