I'm all about interconnectivity too but I think a forced 30-second appearance would stand out really bad. The movies, while all connected in some ways, do stand on their own. You don't necessarily need to have seen Ant-Man, Hulk, the Thor movies, even the first Avengers, to enjoy Civil War. Age of Ultron and Winter Soldier were the most referenced but even that you can figure out by dialogue. But if Ross mentioned the Devil of Hell's Kitchen off-hand, it would be really forced. Really, really forced. Because he's not showing up in this movie and he wouldn't even have a reason to.
Forcing the AoS crew or Defenders into Infinity War assumes that Thanos comes directly to NYC (where the Defenders stay) and plops there for average agents to fire guns at. My assumption is that he'll be multiple places at once, or in space, or something like that. Besides, they'll need Earth's MIGHTIEST heroes. Black Widow and Hawkeye are kind of lucky to be included - this may be a fight for the Captain Marvels, Hulks, Thors and Visions of the world.
And one last point - including the Defenders in Infinity War would throw the shows out of whack going forward, tone-wise. "Remember that time we helped save the galaxy? That was cool. Meanwhile I can't pay rent, and someone just knocked me out with a 2x4 board!" They're local heroes with down to earth problems - it'd be hard to take them intergalactic (in a way) and then back down to earth. It would change the game.
Now, I'll be as cheesed as anyone if they somehow show up or get a reference, but as of right now, they're better off leaving the street-level heroes at street level, the international heroes as international heroes, and Agents of Shield on ABC.
Besides, IW already has enough to juggle: Iron Man, Pepper Potts, War Machine, Hulk, Black Widow, Thor, Jane Foster(?), Erik Selvig, Nick Fury, Loki, Hela, Valkyrie, six Infinity Stones, Nova Corps, Starlord, Gamora, Nebula, Drax, Rocket, Groot, Yondu, Mantis, Captain America, Winter Soldier, the Sokovia Accords, General Ross(?), Black Panther, Ant-Man, Wasp(?), Vision, Scarlet Witch, Spider-Man, Hawkeye, Falcon, Tony and Cap's friendship, Doctor Strange, Mordo(?), Captain Marvel and any additional villains Thanos brings with him.
Those probably won't all be there but I'd say 90% are included. Hard to add much more.