When I was "oop north", we had customers would come and buy hay off us. Helping them load a horse trailer like this one wasn't my favourite, and because of the shape you couldn't get that many bales in.
But the loading faff is probably offset by the low cost of a having covered trailer.
If you have the money, though, personally I'd be looking at a pukka sheep trailer such as a CLH. We get 12 small bales in our 8', lid on. I can load it with the lid on (albeit I'm bent over!) - and it's convenient, legal and safe for taking sheep to the abattoir or market as well.
Don't underestimate the damage sheep will do to a trailer left in the field, though. Little bleeters damage lights, cables, number plates...
Oh, and about using the trailer for storage... Any trailer which transports animals will have lots of ventilation, and if you get stormy weather, horizontal rain etc like we do, hay would get wet if left in a trailer in the field, unless you cover all the ventilation gaps. (And tying tarps on creates more things for sheep to damage... lol)