I have two conflicting preferences:
-I favor stock discs over limited run/OOP
-I favor bagging fewer molds when possible, rather than a different mold for every slot
"Boutique" discs in a unique plastic type are an easy way to bag multiple discs in the same mold. I can bag the same mold/weight in Lucid-X, Lucid, and Biofuzion and get three completely different flights off the shelf. That, to me, is the main benefit of boutique runs. The obvious downside is what to do when you lose a unique disc, or beat it in to the point of needing a fresh one. I've found the balance to be in how frequently I use the disc.
For example, I throw Trespasses in stock plastics as my main distance drivers. I rarely reach for the OS bookend in my bag, so I have no problem with bagging a Lucid-X Trespass in that slot rather than something like an Enforcer or PDx.
On the other hand, I also bag Felons. And unfortunately, I throw my Lucid-X Felon more than I throw the beat-in Fuzion Felon sitting next to it. That is a problem, since I'm relying on the boutique disc and therefore decreasing the beat-in time. If I could go back in a time machine, I'd likely bag Firebirds or Ahtis instead since their stock runs are much beefier than stock Felons.
So really I think it comes down to just how reliant you are on that boutique disc. If it is a rarely used complement for a workhorse in your bag, then buy it. But if you think it will be a workhorse, then you're probably better off finding something more readily available and cheaper.