Readyboost is useful even if you have 64GB of memory but most consumer mobos today top out at 8 or 16GB with a rare few at 32, but add (4) 32GB Usb 3.0 flash drives... you now have 128GB of extra space to cache data.. leave your PC running long enough, and pretty much everything will be loaded from cache

The reason being is any Sata 3 or SATA3 performs less than a USB 3.0 high speed thumb drive. Ram is faster, but, as I stated above, it's limited, and even IF your mobo allowed 64GB of RAM, it's EXPENSIVE
The only time I could see RB being useless would be if ALL your HDDs were LARGE SATA 3 or SATAe (10Gpbs) drives... which they aren't unless your RICH... (and I don't think SSDs that big are around yet). The reason being is Sata 3 and SATAe SSD are equal or greater in speed than thumb drives. In this case, ready boost is useless. In fact, it will slow you down. To replace RB, you need LOTS of RAM and HIGH capacity, HIGH speed SSDs.. this is why if the system drive is a SSD, the Readyboost system is DISABLE because there's no point
I have 16TB of space on my computer and they are high RPM Sata3 drives. I have 8GB of ram, and that's the MAX my PC can have. So I stick in 2 32GB USB 3.0 thumb drives for 64GB extra cache.. after about 30-60 mins Superfetch, and me using it, pretty much have every program and every game cached on devices faster than my hard drives. Once everything is cached, my programs and games load about 2-3Xs faster