What is the difference between tearing out the third row to build a platform level to the second row and just folding the third row that is already level with the second row? Only reason I could see is to build storage into the platform that isn't there with the seats folded down.
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I understand people that don't need the seating of a third row, likely most people. I think this is marketed for people that do need the flexability of that extra seating. Bigger families or families that transport other kids/relatives/sports teams etc.
If you want an SUV without a third row toyota makes a 4runner that's still a true body on frame SUV unlike most other manufacturers. 4runner is probably better in all off road conditions due to its size.
If you like driving a big SUV but don't need the seating you have to make compromises. Manufacturers can't market vehicles to individual wants/needs. The biggest need for large SUV is more seating capacity and that's where Toyota aims the Sequoia.