But most of us scatter them around, not in piles, or in jars mixed with chocolate spread.
I am reasonably sure that wild-life take a different approach to dining out than the one we might take. Obviously, you would complain bitterly if McDonalds served your burger in stages from several different places throughout the neighbourhood. But your average thrush, or hedgehog doesn't give a damn. They'll eat anything that looks yummy from wherever it happens to be. It doesn't have to be in a neat little pile. It doesn't even need to be presented neatly, or with a chocolate spread chaser. A hedgehog can eat 50 -100 snails and slugs a night, from all over your garden, and other gardens. If you scatter your poison, then every snail that hedgehog eats from your garden will be poisoned...
I'm not convinced any evidence I can provide is going to alter your opinion. However:
SLUG PELLETS containing METALDEHYDE are usually dyed blue. They are lethal to hedgehogs and other wildlife that eat the poisoned slugs. Place out of reach i.e. under slates or inside short lengths of piping. Dead slugs must be removed daily.
http://www.hedgehogrescue.org/gardeners.html
There's a useful document here:
http://www.cat.org.uk/ihateslugs/slugpellets.tmpl
and here:
http://www.the-piedpiper.co.uk/th11a(7).htm
The main problem is that no-one reports dead birds or other wildlife when they are found, so no real studies have been carried out, so no unambiguous evidence exists. In the end, we have to make a decision based on your best guess. Personally, I see no reason to scatter a poison around my garden regardless of whether it is going to definitely kill something, or may possibly kill something. I just don't think the risk is worth it. Especially when there are alternatives, that are known not to do harm.
You may see things differently. That's fine. But there is no getting away from the fact that slug-pellets are, without doubt, poisonous to mammals and birds, as are slug-pellet poisoned molluscs. Our only argument is over whether we think birds and hedgehogs are likely to eat enough to come to harm. For the reasons I have suggested, I believe that likelihood is pretty high. So I say don't use them.