Originally posted by mathemosI think (but am no expert) that as in the Phillidor Defence (e4, e5 .. Nf3, d6) this has been shown to be too slow for black at the top level. It is a developing move of sorts, as it opens lines for the c8 bishop, but it blocks in the f8 bishop. This bishop (in this opening) probably wants to be on c5, and d6 prevents this, giving white time to develop a third piece or a better centre, and putting him well ahead in development.
According to Wikipedia the Paris Defense in the Italian Game was played in the 19th and early 20th century but fell out of favor. I would appreciate it if someone who knows why could either explain it to me or refer me to a reference that does. The defense goes
1 e4 e5 2 Nf3 Nc6 3 Bc4 d6
My guess would be that it has two disadvantages, one already mentioned above. The second is that the Nc6 blocks the c6 advance, with its ideas of Qside expansion via b5 and a5 and/or supporting d7-d5. This is a thematic idea in the Philidor, making this inferior since it has the Philidor's disadvantage of shutting in the dark sq B while also preventing the Philidor's Qside expansion.