Mobile operating systems are nimble enough these days to get by, but I still want better processing power because we still don't get instantaneous results for certain tasks.
Photo editing is very CPU intensive. I think tablets should eventually become very popular for photo editing because they don't really require the precision of a mouse or stylus, but the options photographers have today are really limited. My MBP's Core i5 already gets maxed out when editing raw files.
Today's Cortex A9s can't even handle editing large jpegs without getting maxed out. The "auto enhance" button on my 4S takes actually takes about 2 seconds to finish, when it really should be instantaneous.
Web browsing performance is another area where mobile devices are limited by their processors. Loading pages and executing JS is slower on my iPad than it is on my MBP--especially on heavy sites like The Verge. Things get even more limited when you're concurrently loading multiple pages in different tabs.
Video editing is also getting popular and it's almost entirely CPU bound. I don't do much of that though so I can't share any experiences.
So while I agree that performance is sufficient for many use cases, there are still several areas that aren't there yet.