Re-read that wikipedia entry, though: Ð and þ were replaced with th. Besides, "eth" represents the hard "th" sound (in "them") while "thorn" represents the soft "th" sound (in "thin").
Jeff japhy Pinyan,
P.L., P.M., P.O.D, X.S.:
Perl,
regex,
and perlhacker How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart
That both eth and thorn were replaced with th in english by the Normans doesn't change the fact that the eth is the voiced version, while the thorn is the silent version. This distinction is still visible in the IPA symbol.
I believe this distinction also shows in the hip spellings of "the" and "that" by "da" and "dat". I'd say that rappers would vote for Unidecode's decision ;)