The customer would be FAR better off to get a standard 10 year shell warranty than a Lifetime*-6 year warranty (IMO).
Not if it breaks in the 11th year. 
I agree that it's a sales ploy, and I would be tempted to just let it go and get a new one after 11 years, but then again, I would hope to get at least twenty years out of my investment. So, it may not be ideal, but there is some recourse to getting it repaired if it is a structural failure of some kind.
Let me rephrase, "customers (plural) overall would be FAR better off to get a standard 10 year shell warranty than a Lifetime*-6 year warranty (IMO)".
Sure they'll be someone who has a spa that fails in year 11 but what about those who have problems in years 6½, 7, 8, 9 ... and think they're fully covered only to find out they need to ship their spa to the factory. Add in the fact that most still own the spa at year 7 but by year 11 a lot have moved on from the spa.
This warranty l;anguage was chosen for its up from sales benefit knowing the term "lifetime" would give give a false sense of security beyond its true meaning, not from a "lets cover the owner best" point of view (IMO).
