Do my panels need Double Pole breakers? If so, why?

VanHausen

So - apologies for this being a double post. I put it up on "advanced technical" forum but then realized this is probably the more appropriate place...maybe? Anyway I'll leave it to the mods to put it wherever they feel best.

So for some years now, I've had all my parallel panels on my van, protected against a short circuit dump by running the positive of each through their own 10A breaker, just before the controller

Today, I had someone inform me however, that this is a No No - all floating panels (ie negative not grounded) in strings of 3 or more must have Double Pole breakers (ie on + and - ).

When I pressed him on why, got the usual "Er....um...safety...best practice...regulations say so", but nothing in terms of a technical explanation as to WHY you need both lines cut?

I've been hunting all day and can't find anything conclusive, jut a lot of equally vague references about how this is the done thing with 'ungrounded' panels. 

Can anyone explain the actual hazard mechanism behind having only a single breaker per string, on the positive line? 

Thanks!