When I bought my used 06 GX, I wondered about the same thing. I gave it a little thought and went ahead with purchasing the Lexus Platinum warranty. For me, it gave me piece of mind for the next few years that any major trouble I have with the vehicle won't cost me an arm and a leg to get repaired.
Just last week I took her in for the 60K service and the service advisor called to say that my radiator was leaking fluid and I had to have it replaced at a cost of $1000. I'm skeptical in nature and doubted this was true since I hadn't seen any indication of a radiator leak in my garage or driveway. The advisor assured me it was a small leak and only under the "intense" inspeciton of the technician it was found. He guessed that within the next 1000 miles it would have gone completely out. So when pressed to approve the repair I asked if it was under warranty and it turns out it was. So, buying the warranty has almost paid for itself in first repair.
I guess it comes down to you and your comfort level....for me? I'm glad I bought it.
JABDAD