USB OTG is not just about add flash drives. Some ppl like to add external USB keyboard and mouse. Some like to add a game pad etc. Without USB OTG, it is not possible. If I'm not mistaken, there will be HID support in WP8.1. So add USB OTG support for peripheral devices shouldn't be that hard. However, flash drive will be an separate issue as the usage of the SD card today is still quite limited. Without a File Manager (another hot topic), it will be very hard to see much usage of USB flash drive.
In fact, stock Android OS doesn't have USB OTG flash drive access either (specifically, it allows you to plug in but will not mount the drive automatically). It leaves OEMs and 3rd party apps to bridge that gap.