Yeah I don't get it either.
Sedevacantists have no power to bind someone against their will.
We prefer to let people come to the truth without pressure. But the "floating trads," as I think you call those who take no firm stand and fluctuate between SSPX, Vatican II and sede chapels, will still have to face God one day and give an account of themselves.
For myself I would rather be a hardline sedevacantist as it is the only consistent position. I think the other positions all have an element of lukewarmness and cowardice, of trying to cover one's bases, of -- flatly -- compromise. But if someone in my chapel wants to duck out to SSPX one weekend, that's his affair. What am I going to do, lasso him? You can lead a horse to water but you can't make him drink.
I will say that the few people I know who are "floating trads" seem much more tormented internally, unstable in their faith, than the pure sedes. People who become sedes have very high degrees of faith because of the peace of mind they have, the satisfaction of seeing the giant puzzle come together.