Good question.
Maxxiss are the only ST tire I will use but they are not perfect (of course).
I've had two of them suffer a tread separation but they both could have been my own fault.
One had hit a large curb hidden behind a bush and then the tire shop noticed a tiny tread separation a couple of years later.
The other one had gotten a nail in it and the tread separation happened several years later exactly where the repair was.
Neither tire suffered a catastrophic failure.
The first tire was covered under the Maxxis 5 year warranty (it was 2 weeks short of the 5 years) and they paid 90% of the new tire cost. The one with the nail was on me.
So assuming the extra warranty would have covered the second tire, I would have just broken even buying the warranty.
I'm thinking it's not worth it myself.