We have a Panda Pop with the three seat rear bench as an option - and yes it is registered as a 5 seater with DVLA. Frankly I can't imagine why anyone would not go for the 3-seat / asymmetric split / headrests option.
All our kids have flown the nest and I can't remember the last time I had three in the back seat. But it does happen, and when it does I'd have been in a fix without the extra seat. Also why hasn't the two-seat bench got head-restraints? Yet another silly cost saving measure.
After that rant, I also have a 500L MPW Popstar which doesn't have the third row of seats. That rather flies in the face of what I said earlier about extra seating. But I've never felt the need to seat seven and the fold away seats take a chunk of space out of the boot - which is truly huge with the stretched body and lower boot floor.