For my phone usage, the only reason I can think of to want a dual core would be if there were specific apps that required it that I wanted to run...since virtually none exist at the moment, that's a bit "what-if" game...
My phone use is, in rough order of importance/frequency:
3. Podcasts/stream FM
4. Social (FB, Twitter, etc.)
9. Videos (Netflix)
10. Various utility apps (e.g., shopping list, Droid Root Explorer, etc.)
I see no need for a dual core in my use pattern, and believe that the majority of phone users are in a similar situation...dual core is marketing, the next cool feature, and I'd be happy to have it if they solve the heat and other issues, but in no way worth waiting for for me or the majority of smartphone users.