If you're talking of voices in a mod - even in portal 2 - It helps with the immersion. Building nice testing courses is good, but if you have several voices to drive the story it also helps.
We've got GLaDOS and even a fully voiced companion sphere who will always have something to say. I was thinking about making a portal map when I saw iJustine playing. You might think why that someone who is being hated so much by portal fans would actually be an inspiration for a portal mod but there you have it. If you listen carefully to all her confused banter troughout the first 11 chapters (that's how far I got rightnow), can't you just hear another "corrupted core" emerging ?
However I wrote her but didn't get an answer back so I need to redo all the voices to avoid any sillyness or conflict later on.
Next week I'll try to pass the lines on to the same voice who does glados in my campaign. The advantage of that is that I don't need to sift trough tons of video's anymore to get the greatest voices. I can also ask more specific things.
I hope that I can sort it out very soon so that the mod is a step closer from being finished.