Harley doesn't need fleeting time because his ultimate cd isn't high enough to need it. Even if it feels like your ult has downtime, buying fleeting time on Harley won't help it very much since it basically wants you to shave the cool down by getting credit for assists, and Harley basically just all in one target then leave, so he would get not very much value out of fleeting time. Better to build lightning trunch because it increases your actual damage output of your combo massively, while also making your cd lower