Excellent point. I don't see what is more special about a Messiless Spain compared to Prandelli's Italy.
Spain might have won the last euro, the last world cup, they might be full of players who won the last CL, they might be having a 17-0-0 record in the WC 2010 and EURO 2012 qualifiers, all of that achieved without Messi, but as you said...c'mon. Three years ago they failed to beat in 120 minutes the WC holders who practically had the same team that won the WC only 2 years before that.
Italy, as everyone can check, managed to beat the Faroe Islands (twice!), Estonia (twice!) and Slovenia (twice!) in these current qualifiers and that's as good, perhaps a little better, than what Spain is doing.
Italy have nothing to fear. They are (much) better than this Spain team without Messi.