OK, I take 8-cored Galaxy S4 and other dual-core phone, both 1.6GHz, then I get exactly same score in 3DMark.
Now, how stupid I was I paid like 3 times more for the galaxy S4!
I could get the cheaper phone that works exactly the same (at least 3DMark says so).
Well, it depends on how actual games behave. If they are programmed similarly and do not make use of the additional cores (which I guess might very well be the case, since devs usually do not spend much time on optimizing apps for hardware which is not widely in use yet), then yes - you COULD have just bought the cheapo phone and gotten the same performance in those games. Or the other phone which costs a bit more than the cheapo phone (but still less than the S4), which also has a dual-core CPU but a slightly faster GPU, which gives it a higher score in 3DMark than the S4.
Like I said, it depends. It would not surprise me at all if in actual daily use multi-core monster phones make no sense at all (yet), except for bragging rights.