Dual-core phones keep rendering the page with one core, while the other is executing a script, speeding pageloading and overall performance times significantly. ARM-based chipsets are also capable of what Intel calls HUGI (Hurry Up and Get Idle), boosting the cores when something needs to be executed quickly, then efficiently throttling down speed to preserve power.
Moreover, we can rest the case with Adobe Flash and performance with the dual-core chipsets – if decently coded, one can browse most heavily Flash-ed pages without a hiccup, making Apple all the more stubborn not to include support for it in its mobile browser. We can turn it off in Android, but with any dual-core phone you never need to. All in all, the browsing experience itself is worth getting a dual-core phone.