As a Canadian member of both MEC and REI I've long thought that more co-operation between the two co-ops would be mutually beneficial.
In addition to the reasons you give I'd add that there would be economies of scale. Currently both MEC and REI offer house-branded products. These are designed by each co-op and then sourced individually. It would make more sense to use a common design then combine buying power. This would improve the quality, increase the feature sets and keep prices low.
As it happens MEC's history starts with REI. Back in the 1970s a few students from University of British Columbia in Vancouver would drive down to Seattle to get quality climbing gear at reasonable prices from REI. Their colleagues and friends then asked them to pick up gear on their behalf. Soon the volume of this informal business got out of hand. MEC got created to meet the growing market for outdoors gear. Word got out and MEC soon expanded to offer products by mail order across Canada as well as opening a few stores in western Canada. That's when I became a member and started buying almost all my outdoors stuff from MEC.
Of course now it's too late to explore this concept further. While there are petitions, fund raising and court challenges on how the sale of MEC was conducted (specifically for the reason you cited--members weren't consulted), it's likely the sale will go through. It would be an understatement to say that I'm disappointment with that result.
It would be interesting to know if REI considered buying MEC when it was up for sale and if so why they didn't.
one or more gifts or other benefits from the co-op.