I know! Why the heck would anyone want to do that?!? But, what if you wanted two brightness levels--levels that extended beyond the dynamic range of the "256-Step Programmable Global Current Gain" feature?
Well, you might switch Rext in and out of circuit. The datasheet seems to imply that both states are sanctioned: Tables 7.5 and 7.6 include an IDD case where Rext = "Open".
So, is running the TLC5916/17 device without an Rext connected, in fact, an approved mode?