The BNK token is not liquid and widespread/established yet to generally serve as collateral. So it seems quite normal that the native token is not supported yet to act as collateral.
It is not liquid because the project attracts no interest ( beyond the ICO )... Other lending projects stuck to their own coin (but not exclusively ) right after their ICOs and allowed it to be used.
Now how about the Top 20 exchange IDCM ( as claimed by Bankera's team, which we have established by now that it wasn't a top 20 at all ) and supposed massive increase in trading volume. Today it traded 0.8 BTC.. Ha ha ha, yep that's compared to a total volume of 69845 BTC total for the same period at IDCM. Zero interest there too.
As for IDAX, 3 BTC volume.... Out of 123000. To give you an idea, IDAX shows 93 coins listed and BNK is ranking at 71 for volume on that exchange. Out of 93, 8 have no volume recorded so that really makes it 71 out of 81. A 130 Million ICO to end up there ?