Because there are two possible situations when you buy a player.
1) Because you want the player.
2) Because the player is good, andavailable for a reasonable proce.
Milan buying Stam fits into the second category. They already had Nesta and Maldini, so they didn't need a centre back. But Lazio was forced to sell and they didn't want Stam to go to us or Inter. So they bought him, but Maldini can't play left back anymore at age 37 and Stam is crap as a right back.
Moggi mostly buys players that we actually need. Cannavaro, Emerson and Vieira were all badly needed at the time. So I don't think we'll spend big on Joaquin as long as Camo is here.