Actually, in modern day DSLR cameras, usefulness of 'closeable viewfinder eyepiece' - sophisticated, with the shutter or the simple rubber thingy supplied with the camera - is close to zero!
First, contrary to popular belief, open viewfinder does not influence exposure, no matter how long it is; it could (but not necessarily would) have the impact on metering only.
At the moment of shutter opening, mirror has already moved to the upper position, closing the light path from the viewfinder.
You can check that if you try to look through the viewfinder during the long exposure: all you'll see will be the dark frame.
Second, to mess up metering, the photographer has to:
- use one of automatic or semi-automatic modes, and
- for whatever reason, be away from the viewfinder at the moment of metering.
So, if you:
- shoot in M, or
- shoot with Live View, or even
- use semi-automatic mode but have metering and shooting separated (C.Fn. IV), have your eye on the viewfinder when metering and press Exposure Lock before leaving the viewfinder
you have nothing to worry about and you don't even need to close the viewfinder.
But, instead of trying to explain all of this to the new camera owner, it was easier for Canon just to throw into the package the stupid 'rubber thingy' and call it a day.