Is it not the case that in an RTO calculation the effect of T.R's is not considered? They are a bonus and deceleration is calculated using brakes and anti-skid. For the brakes to have their correct effectiveness the ground spoilers must be deployed.
While it is true that brakes will lose effectiveness on anything other than clean & dry, this is taken into account via the non-too accurate, (especially when contaminated) weight reduction calculation.
The brakes are generally very effective and thus there is a rapid deceleration causing the T.R's to lose effectiveness as speed reduces. Perhaps this is why they are treated as a bonus, except on slippery surfaces, when you would most likely be in the lap of the gods anyway.