Derek had some good contributions. His I like live with the band is their cover of Funeral For A Friend/Love Lies Bleeding, they made that song rock. Studio has to either be Anna Lee, Trial of Tears, and Where Are You Now?.
What they were during his time with them was interesting. Although, fun looking, it was a time of struggle for the band. Having to, in a sense, rediscover the band, find the new direction the band must take. And once they did, LTE played a major part in it I feel (although it wasn't intentional LTE was going to include the members it did, I wish it was how MP intended it to be), they got JR to join and came back with a bang.
Although it was short, I feel Derek was the necessary person they needed to help them continue on as a band. He brought a personality that is energetic, watching him on Five Years In A Livetime shows this. Yet the music they made was either hard rocking or Moody.