Q: Do I still need iPhoto Library.migratedphotolibrary?

Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support

A quick synopsis of the support article that Kappy referred to is that this file is a "virtual" image (almost like an alias) in that it doesn't duplicate the entire file, but just points to it. It also enables you to still use iPhoto or Aperture for the migrated photos. I imagine this capability goes away if you delete the file.

It's a bit misleading that Finder displays this file as taking up almost as much hard disk space as the Photos file when in reality it does not. Are you listening, Apple? I'm wasting my time researching this issue for the same reason Roger Barre asked the question...

Are you listening, Apple?

Not unless you tell them - they are not here - to reach them use the feedback page - https://www.apple.com/feedback/photos.html

LN

Since I have no intention of ever going back to iPhoto, do I still need the migratediphotolibrary file?

Photos will work without the migrated iPhoto Library, but I would not burn all bridges before you have checked each and every photo  if the migration went well.

At least keep a backup copy of the iPhoto Library.  In sone cases the migration to Photos needed to be repeated because the migrated library had a corruption that could only be fixed by rebuilding the original iPhoto library in iPhoto or iPhoto Library Manager.

Also, you may want to use iPhoto to add location data to your photos or make adjustments that Photos does not yet support.

Léonie makes some important points about the possible need for a backup copy.