i think they are a great idea. Alot of funeral homes really capitalise on grief. many people don't really care about the fan fare and fancy expensive coffins, but if you have to choose one for your loved ones, you feel like you can't be cheap, so end up paying more than most people would care to pay.
my FIL pre-paid his funeral, but still they put the hard sell on after he died asking if we needed this and that, and "someone" siad "yeah, ok" not realising that they were all (unecessary) extras that we had to pay for.
quite frankly, i'd be happy to be buried in a recycled cardboard box, because i'd rather not waste the resources when i'm gone, so i'll pre-pay to make sure perfectly good wood doesn't go to waste on my behalf.