This really depends on the knowledge you need. You can build lots of experience with Ogre, or whatever, and then discover that the company you want to work for has no use or need for this knowledge. Instead they would have wanted you to be knee-deep in your algorithms and design patterns, which is knowledge you can perfectly gain and apply in a Unity3D context. If lower level framework knowledge/experience is required, then it is a different story. But new/extra knowledge can always be beneficial at some point of course. I like Ogre:)