Most desktop performance sensitive apps are properly multi-threaded (video and audio encoding, 3D rendering) now or will be within a year or two (games). For the rest, I have a desktop using a quad-core 3.7GHz Kaveri and a laptop with a quad-core 1.4Ghz Temash : for regular desktop usage (office, web), I don't feel there's that much difference between the two, even with things like Google Docs in Firefox. I got a much better responsiveness improvement going from magnetic disk to SSD.
I'm sure your mileage will vary but in 97% of the situations, I'm pretty sure the above is true. And the remaining 3% only affect a small minority of users.
To be back on topic, having a Kaveri, I'm really looking forward to see what HSA can bring to the table, especially for video encoding. However, I don't really understand right now what will be needed to take advantage of it.