I think the reasoning behind signing rossi is to have him play the false 9 role when messi is rested are perhaps pep employs messi in a different tactical role in a few games next season, has he is all about evolution of the team season upon season.
I can see rossi being very successful in that role he has the touch and vision to link up very well with Iniesta and Xavi in midfield i think.
for example playing Rossi in the false 9 role and messi wide left would have dealt with the Pepe factor in the 4 classico's, Rossi in that hole and messi wide left would have put that tactic out the window IMO, has they would have had to send a second man to help out marcelo on the left to deal with messi, which would have freed up space for the rest of the team. while when he plays in the middle against a well organized team and good a Dmed, the opposition can pretty much maintain their shaped by playing a 4-2-3-1 formation but still allowing the DM to track messi, has the areas they operate in overlaps, thus their shape is maintained.
------------------Xavi-----Iniesta
-----------------------Rossi
>>>>>>>>>>>>Pepe
---------Messi-------------------Pedro
>>>Marcelo + one
not the best, but it gets my point across the other DM or center half will have to shift left to cope with messi, freeing up much needed space in the final 3rd. for Villa/Pedro and a runner from deep.