Dave: I believe the second isolation switch on the roof is a common standard.
In the AC distribution industry: it's common for lineworkers not to trust any pole-mounted disconnect (ie levers on the side of poles) or over-head-wire grounding clamps unless they can actually see them. Too many situations where someone has fiddled with them whilst they are working, or thought they were done with the job. I suspect it's the same when solar workers are on the roof: if you can't see the isolator, then it doesn't exist, and/or the customer has forgotten about you and is trying to get their aircon going again.
Exact scenarios where a DC disconnector up on the roof would help: not sure, other than perhaps back-feeding to the panels? Otherwise it's just as dangerous up there with or without the isolator, given that the panels are the source? Perhaps moving the feed from the roof to somewhere else in the house is expected, eg when people later change their systems to use big batteries.