I really liked how they tried to use the sentinels during the Kang Dynasty storyline.Not necessarily that they exist in general, but that governments would mass produce them and then only ever use them against one specific group rather than all the time against any enemy. If they can constantly make new ones to put against mutants, they should also constantly be shown in the first waves against things like alien invasions or super villian attacks.
They didn't help, and Kang was easily able to hack their systems and take them over, but the idea they they would even launch a fleet of sentinels to stop somebody trying to take over the world was a nice idea.