He's playing keys on this cut.
The harmonies are better because he was a way better singer than Cain. (Better player, too.)

That Infinity/Evolution/Departure stretch with both Rolie and Perry on vocals is pure gold.

After Greg left the band, too many song were written just to showcase Perry's high voice.
And, Cain is not a good keyboardist. Lots of players (including me) could run circles around him with my right hand in the back pocket.