Nice quote G². There is a lot of over-excitement surrounding fragmentation. Maybe it's because people 'know' that it is important to keep disk files defragmented?

The situation in SQL Server is quite different. Sure, if the majority of your queries involve big (range) scans on covering or clustered indexes, which invoke SQL Server's read-ahead mechanism, then yes, fragmentation can be important (much less so if your I/O system is RAM- or SSD- based!)

Read-ahead is only invoked on large scans where the data pages required are not already in cache. So, for many systems which either do mostly repetitive single-row operations, or which have enough buffer pool to keep the vast majority of the working set of data in memory, defragmenting offers little benefit, other than perhaps making the DBA feel better.

Paul