Until 4PM, you've also been working on things that are value-adds on some level. Your choice not to make use of the small time windows (which aren't actually so small) is your own choice, not a direct function of the office.
(Not that this is necessarily easy to do, or perfect, but I think we can accomplish more than we think in this manner.)
PS: And when you have 15 minutes to work it's next to impossible to figure out how to fix 392 without also creating 357 yet again.
Note that of the distractions you describe, a lot of them are things that you would have still had if you were working from home. The emails and the IMs are still there, although I guess they might be more ignoreable since you don't have the threat of the person showing up at your desk. :-)