The issue or defence constantly served up is that these consultancies didn't directly benefit the client, the "no sporting advantage" defence." It's why the bar should be and is elsewhere that there should be no "apparent bias" inferred rather than actual demonstrable bias which is enormously difficult to prove.
Basically if you're doing work while an MP it should not only be whiter than white but look like it too, as any suggestion or hint of special access or favours brings parliament as a whole into disrepute.
As an example, who in their right mind would pay Chris Grayling for advice? Painfully obvious this is about access, and insider info, which in any reasonable system would be verboten.