well some of the reason why you may seeing people suggesting i5 over i7 because:
1. price to performance ration is very good. aka it's a cheap powerful quad core cpu.
2. Some people may not need the power of the i7 and the i5 will be more beneficial.
Here a pros and cons list i made on an old thread. This may help with understanding the reasons behind getting intels powerful cpus.
Well it depends on what you'll use it for. If it just going to be used for email and web surfing then i would say I5. If it going to be gaming that only going to use 1 graphic card the the get the I7 8xx's. If your going to need 2 graphic cards for a game and/or need max ram bandwidth for memory intensive programs then the I7 9xx's cpu's.
Here's some pros and cons to each CPU series.
I5:
Pros:
Cheap cpu.
Great turbo
Will get the job done for most to all programs.
Runs cooler than the I7 9xx's
Can run faster 1333 MHZ ram than the I7 9xx's.
Cons:
No HT
No triple channel memory
Doesn't have full dual 16x bandwidth for 2 video cards.
Will not be upgradable with 6+ core cpus.
I7 8xx's
(pretty much the same as i5's)
Pro's:
HT
Can be cheaper than the 9xx's (depending on locations and store)
I7 9xx's:
Pro's:
Triple channel memory
HT
full Dual 16X video bandwidth.
upgrade option down the road to the Core i9 when you need 6 cores
Con's:
Can be expensive. (depeding on location and store)
can be very hot if not in a well ventilated case.
Can only run 1066 MHz ram unless over clock to fully use cheaper 1333 MHz.
not as powerful turbo as the i5 and i7 8xx's cpus.
Well i hope this helps.