We had a "hoarder house" burn to the ground about a block away from me a few months ago. I've never seen a house go up in flames so quickly. The fire dangers are very, very real.
On topic...
The reality is that a house that shows poorly sells for less. If I bring a buyer to a house that's loaded with trash, I immediately have two serious concerns. First, if they can't clean up their junk, how much upkeep/maintenance have the sellers been doing to the property? Second, what kind of vermin problem is my buyer going to discover? Both are entirely legitimate reasons, I think, for offering a lower price than if the junk was simply not there.
If the sellers aren't going to be cooperative, then I think at some point the onus does fall on you as the agent to do (within reason) what you can to make the house presentable. The seller may or not may appreciate the effort, but I do know agents who have wound up vacuuming/sweeping/mopping, making beds, or even doing complete trash-outs themselves (or at least paying for it out of their own pockets). Is it annoying? Absolutely. Does it help sell the house? Probably. I would consider this to be going well above and beyond the call of duty, but I like to think that it really helps grease the referral wheels...
Of course, if the sellers are outright resistant to doing any of the work themselves (or to your "interfering" with how they live by doing anything yourself), then there's really not much you can do. However, it's still your responsibility to explain to them how this is going to impact the sale of their house.
(I might also try to explain, in as politely a way as possible, the potential liability of the owners if a potential buyer or their agent were to trip/slip/fall as a result of the clutter.)