There's a little bit of conflating
want with
need. You
need twice as many GTs in order to play the same number of terminals. Does that make you
want them more?
Early on somebody was talking about how having too many Ghost Towns could hurt you, and that is not true.
Well, I have to admit, I was leaving out an important implication, but I'll state it explicitly now: Opportunity cost. Too many Villages also cannot hurt your deck. However, taking many turns buying nothing but Villages, while it doesn't hurt your deck, does hurt your game, and there's even a name for a player who does that.
You always have to factor in how many of something you're going to need to kick off, and how long it's going to take to get it. Peddler is a great card, but only in kingdoms with +buy where you can pick up two or three of them at a time, or pick them up for free along with your other buys. Getting several cantrips or sifters and then buying Peddlers one at a time is too slow to be useful. So there's the "want" issue again. Sure, you want as many Peddlers as you can get. But you have to get them somehow.
Likewise, GTs may be cheap, but you still have to get them. If you have to buy 6 GTs for an engine that would kick off just fine with 3 Villages, where are those extra 3 gains coming from? It's more money, more buys/gains, and potentially more turns to get them. So "having" too many GTs doesn't hurt, but the process of
getting them could.
In fact, I'm willing to bet that's the whole reason for the gain-to-hand mechanic. Perhaps it was originally playtested without that mechanic and found to be too slow and needed some acceleration?