Rather than comparing Orchard to Fairgrounds, I think a much more instructive comparison is with the thematically twinned Vineyard.
Orchard is 1.33 VP/action for the first three actions; Vineyard is .33/action/vineyard (or the same with an even split). Yes the top end possible for most Orchard games is quite a bit lower than for Vineyard, and you need things to be some degree of viable to stock up ... but on the other side you do not need an otherwise useless potion, you never hit $5P and have to agonize between a strong action and small bucket of VP, and you need a far less green tolerant deck. Better still, you are far less worried about the other guy sucking up all the copies of the easily spammable actions or of needing to fish for exactly one card for a huge VP swing.
As a baseline I would at least give serious thought to running toward Orchard VP anytime I would consider Vineyards remotely viable. Yeah, there are a few cases where this does not apply (e.g. Rats/Fortress, Knights), but on many Vineyards boards, I would suspect you can do pretty nice with Orchards. I am optimistic that Orchards will prove to be more powerful than Vineyards on an average board - maybe fewer points, but less hassle getting them.
Even when you are not expecting Orchards to be a major VP source, you should remember the tactical options - three Duchesses are more cash efficient than a Duchy let alone adding in just two more Herbalists. Orchards can also make mass card gain setups extremely powerful for megaturns: Tr -> Smason (trash GG) can be 8 points per, and a lot of the normal tricks like Ball, Smason overbuy, Haggler, Bv, and the like can all offer potential for huge VP spike when busting your deck. Remember that in a standard megaturn game, the Orchard VP is just 2 VP less than the provinces and there are many times where it is far easier to pile up actions.
The real interesting play questions are not when is it worth it to get 3 mostly dead cards though, it is when do you swap from power cards to functional, but lesser cards for the Orchard points. At some point you obviously want to grab some Labs instead of pure Hparties. At some point you might want to mix Village and Shanty or Envoy and Smithy. I have not got a good handle on that as game length with Orchard seems a bit screwy - lots of points delays game end, but because it is pure engine growth you tend not to get much mileage out of attacks or green slowdowns. Knowing when to start picking just how bad of triplets will likely decide an awful lot of high skill Orchard games.