Hmmmm. The only problem that really jumps out to me is the DIY guy, who isn't familiar with stranded wire, making up high resistance connections. I have already seen some photos on another forum of nitwits who have "reduced" the wire size of stranded wire to get it to fit into a connection by removing some of the strands.
I have scary visions of some mentaly deficiant H.O. splitting the strands or something stupid like that.
I work in industry and I must say that I greatly prefer stranded wire to solid. Mainly because it is more flexible. You try pulling 7 #12 solid wires though 2 90deg bends in a conduit.
I am sure that the NEC stipulates solid for a reason (I have no idea what that reason is.)
But, yes, I think that it would be easier to work with. And I do think that it would be a good idea if the person has the proper training, but we both know that won't happen.
"How hard can it be? Besides, my first cousin is an electrician".........BBBZZZZZTTT!
Fine, they burn down their own house. But hopefully, the fire investigation crew can prove that the homeowner did in fact tamper with the wiring. If not, you're the last person who touched the wiring, because the homeowner's last invoice says so. Now it's you paying the piper, instead of the homeowner.
With the litigious society we live in today, we almost need to put tamper indicative seals in place so we can prove if somebody has been goofing around.
I have seen 10/3 stranded. I've also seen 14/2 without ground, it had a nylon string in the ground area. I think the main reason they don't make 14 and 12 stranded NM is the dificulty in terminating on devices. It would come in handy for certain situations.
There is a cable that is similar to NM (Romex) that is used on boats. It is UL listed, rated at 600v AC or DC, 105c dry, 75c wet. I would guess that it is not listed for use in buildings, but it is interesting stuff. Look here http://www.cobrawire.com/boat/ul1426.html#round WireWrestler