Actually real world usage is not negligible, as the 5S has capabilities that depend on the power of the processor, while the 5 cannot do these things:
- 10 pics/second burst
- touch id
- slo mo video
So while I agree, opening an app is virtually the same on all fronts, 64 bit processing provides extra capabilities not able to be done on 32 bit. The extra bit of speed, or pop, if you will is also nice.
In addition the integrated hardware a7/m7 of the 5S allows for capabilities above the 5, like what fitness apps use.
All of this translates into a more robust platform, even if you don't see a speed improvement opening up an app.
The 5S vs 6 is less clear cut. New hardware on the 6 allows for capabilities not on the 5s. Speed of the a8/m8 would *not* be the reason for an upgrade for most people. Bigger phone, more battery and NFC might be some reasons. Other reasons might be, just because. But you also don't know what else apple tweaked in terms of hardware: ddr2 vs ddr3, fast NAND etc, would make a phone faster....it's not all pure CPU speed.