Plan has some interesting opportunity-cost math.
Most trash-one instances are underwhelming (Raze, Trade Route, Forager) which don't tend to create lean, mean decks compared to the multi-trashers (Steward, Remake, Chapel) which can thin decks down to practically nothing. Plan is somewhere in between.
This event looks like a trash-one at first, but it compares surprisingly favorably to cards like Remake: You open Remake, [whatever] and then shuffle. You aren't playing Remake every turn, you're playing it roughly every OTHER turn, once per shuffle, so you're essentially alternating between trashing two (and maybe gaining one or having enough money left to buy something) and trashing none/buying one. So it averages out to be trash one/buy one every turn.
When you Plan, you're also buying one and trashing one every turn, provided you want a lot of whatever you're buying. It doesn't reduce your deck size, but if you're buying a cantrip, it's effectively reducing your deck size by one. Plus it starts trashing turn 2 instead of 3/4.
If you're not playing a one-card strategy, however, then you have to start buying other things and it stops working. Moving the Plan token usually isn't an efficient use of buys/turns. On the other hand, it also doesn't end up as a dead card in your hand as happens with many trashers once you start drawing it only with cards you want to keep.
So buy Plan if you're building something that doesn't require thinning your deck of all of your starting cards, but could benefit significantly from thinning as you build.