If I ever buy another grapple, it will have a solid bottom. Like other have stated, I thought it was important to have tines on the bottom so debri would fall out. So far, in over decade of using it, that has never really been an issue. I don't scoop up any dirt when picking up trees or branches.
My reason for wanting a flat bottom is to carry dirt on my way back to the tree I'm working on.
My most common project with my backhoe is clearing trees. I dig them out with my backhoe, cut them up into lengths that I can carry with the grapple, and bring them to the burn pile, or firewood area. Once I get the roots ball out of the hole, I need to fill it up again.
I've tried carrying dirt with the grapple, but most of it falls out through the tines, like everyone says they want to happen. What little amount that I get there is almost not worth the effort. If I make four or five trips to haul off a decent sized tree, I could also make that many trips going back there with dirt in my grapple if it had a flat bottom.
The way I do it now, is after I take out the trees, I have to come back with another tractor to haul dirt to fill the holes. Or I have to remove the grapple and put the bucket back on.
The grapple on the bucket sounds like a nice compromise for what I'm doing, but I feel that I would loose too much lift, or abilty to haul the most material possible with my grapple if I dind't have two lids on it.