I think what he's saying is that, by removing the sprue gates after you've assembled the model, you're less likely to damage the part in noticeable ways - if you're removing a gate from the left half of a leg, for instance, the left half will still match the right half when you're done because whatever you did, you did it to both halves... The mating half also provides a small amount of extra protection against gouging the part, acting as a sort of guide.
Personally I don't think it's worth it. The assembled second half acts as a guide, sure, but this increases the risk that you'll damage, if anything, not one part, but two simultaneously... And if both halves have gates in the same place, the assembly may not go together especially well until they're removed. And assembling the halves makes the gate harder to access than if you simply had the unassembled bit on the cutting mat...