I agree that multiple flight numbers for the same physical flight are an annoyance enroute.
However, the importance of codeshares with US carriers is the rule that all US government employees, as well as employees of contractors to the US government, must fly on American flag carriers.
Many large US companies set up their travel website so as to comply with this. For example, as a Canadian employee of a US firm, I went to the company travel web site for a trip from Ottawa, CYOW, to Halifax, CYHZ. The preferred routing was via Boston with a long layover, even though Air Canada has several non-stops.
Code sharing provides a loophole through this requirement, which somewhat levels the playing field and also gives the US unforntunates more travel choices internationally.