The idea of the bonding is 99% to make sure that the differences between the action tolerances and the rail tolerances are zero. Some added sheer strength is added to the end result, but that is secondary in purpose.
In the perfect world this would not have any benefit, for the rails mating surface would be an exact mirror image of the actions mating surface. Unfortunately this is not the case. I see many rails that have more stiffness than the action they are intended to be used on, sometimes there is several thou difference in the height of the action, from front to rear. Or they manner in which they are machined leaves gaps in the mating surface areas Shims are 1 crude answer, bedding compound is a far more accurate solution.
To re tap to 8x40 , is in my opinion , of little value, unless holes are stripped, or huge optics are being used. The potential for bending a rail or distorting an action from bigger screws on an imperfectly mated scope rail/action is increased due to the greater torque the 8x40s will take over the 6x48s that seem to be the industry standard.
Retapping to 8x40 makes good sense however if really heavy optics are to be used. I in fact use 10x32s to hold the rails onto my 50s, as the optics used there tend to be really huge.