"personally I don't think"? This is tech, there's no room for belief here, particularly with the hard facts easily available. Just go to e.g.
www.storagereview.com and see for yourself how wide the performance gap from IDE to SCSI is - as soon as it comes to multitasking and file/web/database serving environments. On a single user, single task machine it isn't worth the effort, but as soon as we're talking serious workload on the disk storage, you'll notice. Just see for yourself, storagereview have a very comprehensive and detailed "Performance Database" feature that lets you compare.
And don't even get me started about drive reliability. SCSI drives aren't expensive for nothing.