Then all the ISP would have to do is coordinate with wikimedia to put such a device in their datacenter and save a lot on that precious upstream bandwith, keeping traffic on their local network where it's not costing them as much.
That's a plus, in addition to the free karma.
I would imagine that on average a page is retrieved more than it's edited. So even even in the case of poplar pages, there is potential for saving bandwidth.
But I don't know the actual statistics for wikipedia.
And it isn't so much for the bandwidth saving, more the saving in CPU time in rendering the page up from the Wikitext source, which can be substantial. Some very big pages used to take 2-3 minutes to render because Wikitext is a rather slow ugly programming language pretending to be a document format. (A lot has been sped up after a lot of the rendering guts got moved to Lua.)