From a specific very high profile brake manufacturers site..
Finally, under an OEM bias condition, the rear brakes only contribute to about 15-20% of all the braking force the vehicle generates
Depends if you want to believe a "shop" or a component manufacturer that works closely with actual vehicle manufacturers
Either way it matters not
From my own experience (in competition) when I lost the rear brakes completely, the only way I noticed was because the pedal sank a bit further down than normal. The car still stopped the same up front, but it just wasn't as balanced under braking and nose dived more. It also made left footing it around corners a lot more difficult.
So its correct they dont do much, but what they do do is done for a purpose