Every computer on the market that can read Microsoft Word documents can also read plain text without anything being installed.
"you're asking people to optimise a standard behaviour"
"Standard" according to whom? Some rough numbers from Googling from a couple of minutes:
Number of personal computers in the world: 1 billion Number of copies of Microsoft Word sold: 100 million
So, no, having Microsoft Word (or at least a legal copy of it) is not standard.
"for a marginal gain"
Again, according to whom? What is the relationship here? If he is your teacher, boss, prospective boss, or customer, the gains might be anything but marginal. It looks like he is a computing teacher, actually. I've had plenty of teachers who required work to be formatted in a specific way from which you deviated at your peril.
The assumption that everyone has Microsoft Office running on a Windows machine was never true, but in today's mobile world it's not true with a vengeance.