A defensive midfielder (holding) midfielder is a must for every good side simply because if youre any good then youll have specialised attacking midfielders (such as Lampard) who you will want to free up as much as possible to influence the game in the final third. The idea that these holding players are in some way protecting the weaknesses of others is a very one-sided argument as you never hear how, for example, Makelele is only made to look good because Lampard scores 20+ goals a season. Defensive midfielders are crucial for any decent side, but they usually only shine in good sides as highlighted by Mascherano last season and the contrasting form of Dietmar Hamann in Pearce's City compared to Sven's.
If you look at most top sides from around the world they are all operating with combinations of defensive and attacking midfielders. Genuine all-rounders (top quality ones anyway) are very rare and, as we have seen with Steven Gerrard, their versatility often works against them in how they are worked into formations and tactics compared to those players with a definitive role.