Hi,
the idea behind host powered was simply to save one power supply (for 5V in the case of a FT2232D).
I thought that for such small currents you will get a voltage regulator for something like 30 cents?
How do these small devices provide USB host at all if they don't have 100mA? I thought the standard required at least that (and the chip would need less).
Well, they don't conform to the standard. The manufacturer puts in a chip that could support USB slave, OTG and host but they officially support only slave. By changing the driver in the linux kernel you can change the operating mode of the chip to host. At xda-developers some people did that. But they have to use a powered USB-Hub inbetween the devices because of the missing power supply from the host.
Of course that won't be a common use case. But if I have an old smartphone that is worth < 20 EUR it might be the most cost effective solution.