Originally Posted by JamesLoughney

Robby - good question - and in a free market I would. However, booking through the Delta website for the route used by mrcross and myself - LHR/ATL, a route I have to use - results in a range of options, of which 90%+ are Virgin flights, under their codeshare arrangements, started in 2014.
There is no filter option on the Delta website that allows you to exclude other operators.
A rationale for codeshares is that it increases customer choice. On this particular route, Delta appears to have handed over the majority of their LHR/ATL flight slots to Virgin - no doubt in return for a concession elsewhere.
I continue to book through Delta for this route because they are consistently cheaper than BA, the only other operator on the route.
My beef is this - as a long term Delta Silver Card holder, that card's benefits have pretty much disappeared for me as a result of Delta's unilateral action on this route!