Level scaling would make running old dungeon and raid content feel just like it did the first time through that content. Level scaling in dungeons and raids would revitalize that content, and make achievements for completing previous dungeons and raids mean something.
Finally, level scaling would revitalize world pvp and battlegrounds. Level 90s might think twice before going to hellfire peninsula and ganking lowbies if they were at the same level as the players there. Lets face it, battlegrounds were a lot more fun leveling up or in previous expansions than they are now at maximum level.