Why does the Segwit developer guide recommend to estimate fees on basis of vsize instead of weight?
The advantage of using vsize is that it is a smooth transition from size; every non-witness transaction has vsize equal to size.

All code and infrastructure that used satoshi/byte before, will keep working when substituting size with vsize, and give consistent results. Switching to weight would be confusing - would we be talking about satoshi/weightbyte?