They don't have to sound "alike" for him to be able to perform the material. Sammy and Dave and Gary didn't sound alike, but they could all perform each others material live. That's what you're not getting. Just because you felt Michael Sweet and Brad Delp were different you think it didn't work but they had very successful shows and I never read anything bad about Michael Sweet fronting the band so IT DID WORK.
http://www.youtube.com/watch?v=vdPH3Cl-a2Uhe did a pretty good job here with Stryper and Scholz