I have over 150,000 original miles-it's been a real long run (over 10 years) of holding those and using others. The primary benefits to not transferring them were no expiration and they could be used for better access to a few awards (Hawaii but not Europe since UA did not fly to Europe 10 years ago-this sounds like ancient history, doesn't it?).
The lack of expiration isn't such a big deal now but sure was for a long time.
Caveat: UA has had many previous announcements that original miles would either expire or be rolled over. I know folks who transferred original miles into the regular program, only to see these prior dealines pass without the threatened expiration being imposed. I have read that UA was holding back on imposing new sanctions on original miles for legal reasons. Whatever the reason, if the difference between original and new matters to you, you may want to wait until we are closer to whatever the latest deadline is and see if this one finally has some teeth.