My employer has observed that productivity has not taken a hit with us working from home. If anything, I'm more productive working only at home than working only at the office.
That said, I know some days (if there are meetings and/or a lot of commotion at home) then I would choose to go the office. Ideally, they would give each employee (working with their supervisor) a choice in what they want to do on a day to day basis as we know where we will be most productive. Not sure if my employer would still choose to rent a big office building if employees are only using it 1-2 times per week, but keeping it as an optional work environment would be the best option purely from a productivity standpoint.