Indeed. I think that this will be a big feature so this is a very good point.This is what I would expect to happen. While employers may be making a saving on office space, they are also paying the extra IT costs and won't want the extra bill of paying people to travel to an office.
We had an email around the office a few years ago which suggested that regularly visiting another office for internal purposes, which was suggested as being at least one day every other week (ie 10% of time, let alone 40%), was potentially enough to be classed by the HMRC as regular commuting, and therefore potentially be subject to personal tax liability if expenses are paid. It would appear that this extends to people whose place of work is home but are called into an office every other week and that, even if it were paid as expenses, HMRC may come to consider it as a benefit in kind.
On the other side of this coin the contracts I'm more familiar with are aimed more at preventing employees from doing any work in an office unless there's no alternative. i.e. visiting an office would be by invitation only and one would be expected to go home once finished to continue with work if necessary.