The problem with that, especially in image heavy threads - which is what we're talking about here, is that those images off site tends to disappear after a while. Often a very short while. Making even quite new threads more or less un-readable, with lots of "Image has been removed"-placeholders and/or seemingly empty posts.
If those images were attached, you wouldn't have that problem.
Oh totally. I hate that too. It's just how the internet is though and forcing people to use only attachments doesn't seem like a good solution, at least in my opinion.
I think a more simplified approach could make life easier for mods and create less butthurting among members. Something to the effect of stating that 1000 pixels (or whatever) is the max width an image should be posted inline and any others will be TIMGed by mods. Less mystery, less pain.
Simples!