There is so much stunning eye candy between the cars, the photomode videos (fly arounds of you rgarage cars in the future photomode locations - complete with swaying grass and butterflies hehe), videos o fthe race tracks, HD download movies on GT TV, the driving is just one facet - so replay videos don't impart the entire ambiance and environment ....
And I think it does help I am doing the GT5 thing with the Playseat racing seat, G25, home theatre surround sound with visuals being handled by a 60" 1080p Sony Bravia TV.
If Forza looked half as good with these replay cameras and G25 support then Polyphony would have a challenge on their hand