I guess I can see the point of not wanting to sacrifice one of your Potion buys on Transmute, but that seems to be more a case of opportunity cost. That cost can be mitigated a lot with +Buy, since you can spend your Potion on a Transmute and your $ on some other engine component. That's often a better move than just buying e.g. an Apothecary, I'll wager, unless you have a really compelling reason to win the Apothecary split. So I would say that Transmute needs +Buy to be at its best, but probably you wanted +Buy for your engine anyway.
It's not just a matter of needing +buy. Unless you're willing to get a second potion, you're only getting at most one
-cost card per shuffle. And usually, you really want that card to be whatever made you buy the potion in the first place. They also tend to be things you want lots of, and often times things you need to win the split for (Scrying Pool, Apothecary, Alchemist). By getting Transmute instead, you put yourself behind a shuffle (potion-card buying-wise), which can be a big deal since the cards are things you'd like to use fast. Of course it isn't helped by the fact that SP and Apothecary are crazy strong cards, and Alchemist is pretty strong too when it's cost isn't taken into account, so the competition is usually very appealing.
By the time you're no longer excited about whatever made you buy the potion in the first place (either because the pile is empty or you've already got everything you need), it's probably too late to make good use of Transmute.
There are times when you'll take it, but you really need everything to line up right. I could see myself getting it if I hit
when I'm hoping for Alchemist or Familiar. Transmute is probably better than most $2's if you don't already have any trashing.