Just because those poems weren't written by ancient Persians, does that make them any less good? You've been misled about the source, fine, but if you like the poems, I don't see why this would make you like them any less. Yesterday I found out that the melody to "Don't Think Twice, it's Alright" was lifted from a Paul Clayton song. That doesn't make it any less good.
If part of the value you place in something is the trust that it's authentic, finding out otherwise diminishes the value. If for example I picked up "The Essential Rumi: Poems as Interpreted by Coleman Barks," and "The Gift. Poems Inspired by Hafiz," I would better know what I have in my hands from the very beginning. I would look at them from a different angle and appreciate them from that angle. The thing is though, I'd also be less likely to read them, let alone buy them because I'm interested in the original works of the original authors. By being told I'm buying something I'm not, I've been taken advantage of. By realizing I'm being taken advantage of, my emotional value in the works has dropped to near zero, souringy taste for them and making them near worthless.