It's worth it if you're running stuff that makes effective use of an i7 processor.
...and running that *a lot*.
Honestly, count the number of minutes a month that your CPU is fully tasked. Now multiply that by 0.07.
That's the time-savings a month by going for say the 2.6ghz CPU from the 2.3ghz (according to pure-CPU benchmarks like Geekbench).
Only that time savings is not quite accurate, is it?
Chances are you're not waiting for that handbrake encode or that render to complete. Chances are, whether it takes 120 seconds or 111 seconds, you're doing something else (reading websites, etc), and aren't sitting there staring at it like the endless final seconds of the microwaved lunch.
Chances are you only come back to those extremely CPU intensive tasks, well after they've completed. In such a case, the CPU savings is entirely moot.
I pin my CPU maybe a hundredth of a percent of my usage of my MBP (YMMV). Of that time, I am never sitting waiting for it to complete.
If like me, you let those CPU-intensive tasks sit idle for several seconds/minutes after they complete, then you will notice practically zero difference with a 7% processor bump.
And if that's the case, spending an additional 7% of the *entire cost of the Mac* would be foolish.