A double vapour treatment a week apart when broodless is a 100% mite kill almost every time( but lets say it isnt and a single mite remains)
Given the doubling of mite populations normally, the difference between leaving 1 mite and 30 mites is about 6 months more were your colonies remain below treatment threshold. If you can't see the value in that, take it up with LASI, theyre the ones who recommend it, they and other researchers have far more experience of opening colonies through winter than any of you too scared to have ever tried