I hate it when they do that. All Flash does on the show is run fast. Green's Arrow shoots trick arrows, and none of the characters in DC's Legends of Tomorrow interest me. I'm not the only one who doesn't like it when they do crossovers. I was talking to a co-worker last Monday who told me they were doing another crossover, and she said that she doesn't like it either. They just revealed that there's another Kryptonian, and we have to wait to see what happens with her because of the crossover.

its a very effective marketing ploy and gets good ratings (for the cw anyways)

We're into the third season of Supergirl, and they haven't even said if there's a Justice League or not. I don't know if Supergirl is in the Justice League, but I know Martian Manhunter is, and I'm guessing there may not be a Justice League on the TV universe of Supergirl since Martian Manhunter is so busy working with the DEO. Plus they made it seem as if the Flash is from another dimension. If they want to do a story that involves other super heroes, why not bring in my favorite, Green Lantern. It'd be awesome if all six Earth people who are in the Green Lantern Corps right now and Alan Scott showed up. For those who don't read comic books, here are the members of the Green Lantern Corps from Earth: Hal Jordan, John Stewart, Guy Gardner, Kyle Rainer, Simon Baz, and Jessica Cruz.
Take care and God bless