There has been recent interest in using orthonormalised forms of fixed denominator model structures for system identification. However, modulo numerical conditioning considerations, the transfer function estimates obtained by using these sometimes complex to implement structures are identical to that obtained by simply pre-filtering the input with an all pole denominator with poles the same as the orthonormal form and using pre-existing software for FIR identification to estimate the numerator co-efficients. In recognition of this, the report here provides detailed comment on he utility of using orthonormally parameterised model structures in a system identification setting.