I wouldn't be too quick to blame Chrome. What if each open background tab is running a computation/upload that requires the DOM? What if you're listening to music in the background? Point is, Chrome can't just arbitrarily decide to pause execution and free up memory for background tabs.

Me, I'm a documentation-holic. I need to have all the references for whatever project I'm working on a single click away. Sometimes that will be working with a very specific library, so there's no point in replacing my entire bookmarks bar with all the documentation links for that. I'd normally just leave everything in tabs, and when I run out of memory when I need to launch a memory-heavy app, I'd go to Chrome's Task Manager and kill those tabs. So this is a godsend for me.