To be clear, Stack Exchange buys image hosting from Imgur; images are hosted on a separate domain and don't get any of the cruft folks are complaining about here. This relationship has been in place for years and has been mostly pain-free for a involved.
Wow that sounds strange coming from StackExchange. It seems hard to believe imgur would have a cheaper way to host images than what SE could get, and it's not like an image upload page is that difficult right? I suppose it's one less thing to deal with does it make that much sense? (Honest question, not trying to "weekend project" imgur.)
Images aren't exactly a core requirement; if someone else can handle that without a significant down-side, then why not free up people to work on other things?
Source: I work for and use Stack Exchange.