Game and multitask, just as you do on your PC.
Open a half-dozen tabs, stream music, update feeds, compose an email and text, while shuffling in and out of a game... Many, many people do these at the same time. The first one alone will choke a single core on broadband.
Opening six tabs is solely RAM-dependent.
Streaming music uses minimal CPU resources.
Updating feeds uses minimal CPU resources.
Composing an e-mail uses almost no CPU resources.
I would hope your games don't run in the background - if they do, your battery life will be terrible.
I open six tabs at a time on my HD7 frequently - it certainly doesn't "choke."
An iPhone 4 will similarly not "choke" on six tabs. The only reason a single core Android phone may "choke" is the presence of Flash. Running a bunch of Flash ads in the background (which they shouldn't be allowed to do, anyway) will make your phone lag; neither WP7 or iOS suffer from this issue, however.
Do you want to run a video in the background while playing a game? A dual-core may be useful.
Do you want to encode video? A dual-core may be useful.
Do you want to process images? A dual-core may be useful.
The tasks you mentioned, however, are (at their worst) RAM intensive.
Modern computers never utilize single cores because of marketing stigma against them, not because they are truly "obselete." A modern single-core (think Sempron 130-150) is absolutely capable of providing a smooth experience.