Nah, I'm just having some fun. But you're asking the wrong question.
The question is not one of utility, but of truth. That's something all traditional Catholics can get behind. The question, "why are we traditional Catholics?" can be answered legitimately by pointing to the fact that traditional Catholicism is true, and that God made us to apprehend the truth. So the only question that we should be concerned about when approaching sedevacantism from a non sedevacantist position is whether or not it is a true allegation. Come what may from there.
I have answers-- regarding the practical benefits of sedevacantism-- I just don't think that they're the primary reason that anyone should end up concluding that the conciliar claimants aren't popes. The primary reason is distillable to truth; in this instance, a theological truth that is intertwined with the nature of Christ's mystical bride, an organization which is a visible unity of faith and charity, and one from which those who have severed themselves (from either bond) do not belong.
Even if there were no practical advantages to acknowledging these non-papacies at all, the fact of the matter would remain undisturbed. I think it's important to acknowledge this. If we evaluate usefulness before truth, what does that say about us?
Anyways, those who acknowledge the state of sede vacante have clarity. They are not pre-occupied with looking at what the "pope" says and figuring out what is true or not. They're also not pre-occupied with the sedevacantist question, either, which is something sedeplenists find themselves frequently anxious about. Practical devotions become clearer, because you have a "deadline" after which you know that no pope has canonized a saint, approved a Litany, instituted a change in sacramental rite, etc. You can actually focus on your spiritual life instead of being absorbed in controversies. You can tell your children, "that man isn't pope, so don't listen to him." Now, one could say, "I'm not a sedevacantist but I do all this anyways." Heh.
What do you call a man who doesn't teach as pope, doesn't legislate as pope, doesn't canonize as pope, and doesn't actually do anything a pope does? To invert the old saying: if it doesn't walk like a duck, or quack like a duck, well...
Now of course, one needn't be a sedevacantist to go to Heaven, but if we're distilling things to the minimum prerequisite to be saved, once again: what does that say about us? If a person judges that they can discuss the matter, then let's discuss the matter; let's not focus on pragmatics at the expense of ontology.
Honestly Matthew, sounds to me like you're convinced of sedevacantism and you're asking the age-old (valid) question: where do we go from here? Maybe you can share with us how you think the SSPX, or Bishop Williamson, could restore the Church.