Great question Mark, I'll try to do your question justice!
I was looking at this from two aspects: Performance, and long-term capacity.
While the system does indeed have 42 disks today, tomorrow it may have a need for additional capacity.
So, by choosing a 15disk raid-group, I'm assuring myself not only maximum efficient RG design, I'm also committing to the maximum amount of space.
Also, with his third disk-set sitting at 9 disks (7), it is usually seen that your smallest RG need be atleast half the size of the RG size itsel, by having 7(9) disks there, we meet that criteria. (Although, being that Jan-Jacob did not require being capacity bound, that 3rd plex need not be added until necessary)
Coming back to other possible options, based upon todays available disk.
If I'm guaranteed a maximum amount of disks (42) in my system and never expect to grow it, then a 14 disk RG could indeed work.
But as storage is always growing, tomorrow comes around and I add another 2 shelves to my system, and when I try to grow it based upon a 14 disk RG I would end up with 56 disks allocated to the aggregate.
A 56 disk aggr isn't bad, it fulfills the criteria of performance by giving me appropriate spindles and also provides me a sizable amount of space - but my maximum capacity (bound by the RG) is stuck at 1TB less than the maximum availble to me in a 59 disk aggr (fulfilled by the 15 disk RG). With that in mind (I considered it) I opt'd to go for the best practice for best performance while also being able to fulfill maximum capacity in the long-run.
My sources for this were a calculator and capacity documents.
Hopefully this helped bring some insight into the operation and my decisions around it.
Thanks Mark,
Christopher