'Cos it will sound cack
Adaptors of any kind, on a signal lead (especially such a critical one as this), are about the worst possible bottleneck you can introduce into a system! Avoid at all costs.
However, Geoff, you'll only get the full benefits of BNC, if you do it throughout, by replacing the RCA socket on your Allo Digione or DAC (whichever item currently isn't fitted with a BNC socket), and then fit a BNC plug at both ends of your respective cable.
That's the only way of doing the job properly
Marco.