It could be taken in some countries (UK for example) as false advertising - calling the service Cortana implies the service will use Jen's voice in all regions as she is the voice of Cortana. From the outlay it was assumed by everyone that Jen was the sole voice artist at work on the project - nothing was ever said about different people being used. It's only now that the reverse appears to be true. I don't think Talderon even knew until recently - the OP's link shows a screengrab of one of his tweets and I've double checked on his Twitter feed, and there it is.
Having Jen's voice is what differentiated the digital assistant on WP from Siri and Google Now. The UK voice of Siri is some guy nobody has really heard of, and I can't speak for Google Now as I've never heard it. Having some random person nobody has heard of doing the voice just makes it more of the same, with nothing to make it unique.
If they instead based it on say, the computer from Star Trek, people would expect to have the same voice globally (not possible now I know, since Majel Roddenberry has long since passed away), but that's the iconic voice people would expect - and the Cortana service is no different in that regard. Another example - HAL. People would expect the voice of HAL9000. Or there's KITT from the original Knight Rider. All iconic voices which wouldn't be same with a different voice attached to them.