Just to weigh in a little late on this one.. 

Developmental speed can often outweight a fully-optimized data source.  I.e. if all dates are "datetime" and all strings are "varchar" then it can be faster and simpler to design the apps that sit on top of them.  I'll admit it's not as optimized as it could be, but there's a huge cost savings in knocking a month out of a development cycle that could easily buy a whole rack of SCSI drives.

I would personally only use this type of optimization on something where bandwidth was absolutely critical such as a wireless app with very limited bandwidth.