The principle advantage of have dual sups in a VSS/StackWise pair is with single sups (per chassis) if you lose a sup you lose half your capacity until a failed sup is replaced.
"Two Sups is quite expensive."
Yup, but there is a "third" option. Have one chassis with single sup, other with dual sups. If sup fails in latter, no outage. If sup fails in former, move standby sup from dual sup chassis to replaced failed sup in single sup chassis (i.e. failure time in minutes).
"There must be some compelling reasons to justify costs."
True, but same question and answers might apply to why you chose to use a 9606 pair. For example, a 9410 would provide the same number of line card slots, i.e. possible less expensive than a pair of 9606s. So, if 9410 less expensive, I suspect you had reasons to justify the extra cost for a 9606 pair.