Desirability Reasoning in Grice

On p. 46 of _Aspects of Reason_, Grice proposes a comparison:


We should compare such sentences as

1. Given that the barometer falls, it will probably
rain tomorrow.

2. Given that action A would be a lie, and that
action B would not, B is better than A.

These may be regarded as exemplifying, respectively,
the forms:

1b. Prob (m1; p)

2b. Pf (m1; B better than A)

["'Pf is to be read as 'prima facie'].
_Aspects of Reason_, p. 46

Grice notes some signs of discomforts in trying the analogy between 'pf' and
'pb' too far.:

in practical arguments, unlike probabilistic arguments,
the special connective ... will not disappear in the
conclusion. p.47

But surely that's some minor objection NOT to endorse Kant in his idea of practical reason, right?