ReadyBoost is designed to help when your PC's memory is running low. Low memory can make your computer sluggish because Windows, which needs a place to stash data, turns to the hard drive. Flash memory offers a speedier alternative.
In my 64-bit system I have 4GB of memory and for my uses, even with heavy multitasking, I don't reach this limit. Is there a good reason to still use a spare USB and activate ReadyBoost or will it only be a silly dongle sticking out of my notebook?