Uhmmm...no....
That person was either
A) Very silly
B) Very inebriated
C) Joking with you
I know lots of people that use SCSI and IDE combo.
IDE for big cheap drives for stuff like MP3s, DivX's, and other "media" *cough*pr0n*cough*. And then you use fast SCSI drives for your OS, main programs, virtual memory and games if you have the space.
It's quite effective really. There is lots of stuff on your hard drive that gets no benefit whatsoever from SCSI so unless you have money to burn there is no point in putting it SCSI. On the other hand there is plenty of stuff that can make good use of SCSI. So having the dual setup is very good.