the good thing about that team was that all players in midfield were equally instrumental as they offered very diff qualities and that way complemented each other.
Pirlo's role was obviously important but not any more than that of any of his fellow mids - his presence on the field would have hurt the balance of the team, if not for a player like Gattuso.
Seedorf, on the other hand, provided qualities that are rare in players in general - toughness and extra muscle in defending, coupled with great ball-possession skills and creativity in attack to compensate for Pirlo's lack of physicality and speed. It was the dutchman who provided the balance in that team as without him, that Milan would have been either too easy to penetrate with another creative mid in his place, or too defensive and lacking in creativity with another defensive mid instead of Seedorf.
Kaka, of course, provided the penetration going forward, which compensated for Pirlo's, and later on Seedorf's (once he passed the 30 year-mark), lack of speed and explosiveness.
As someone said above, Pirlo is a very specialized player and needs to have the right players around him, otherwise he can quickly turn into the Achilles heel of his team, especially now when he is no longer in his early to mid 20s