I like to keep the top covered not only because of bird droppings etc. and UV degradation to the gelcoat but also because of UV degradation to all the vents etc.
We had a full cover on our Scamp. For me, it had some pluses and minuses but we were planning on getting a full cover for the 19.
Since I knew that I'd be working 57 out of the 60 days in July and Aug.
and it seemed a shame that the trailer was just going to sit out in the sunshine
while I worked I thought about putting a tarp over it. Then I remembered that when we gave our Toyota Corolla to our niece the cover stayed on a shelf in the garage. On a whim we thought we'd throw it on the trailer and see if it could be used as a temporary cover. I never thought that a cover for a Corolla would ever fit but was pleasantly surprised when it did want I wanted, covered the top.
It has some pluses. The door can still be opened easily without undoing zippers and when you're doing things inside there's more daylight. The thing I like the most though is that it deflects rain slightly outboard and no black streaks under the windows have developed.